Acuan API QuerySet

This document describes the details of the QuerySet API. It builds on the material presented in the model and database query guides, so you'll probably want to read and understand those documents before reading this one.

Throughout this reference we'll use the example blog models presented in the database query guide.

Ketika QuerySet dinilai

Internally, a QuerySet can be constructed, filtered, sliced, and generally passed around without actually hitting the database. No database activity actually occurs until you do something to evaluate the queryset.

Anda dapat menilai QuerySet dalam cara berikut:

• Iteration. A QuerySet is iterable, and it executes its database query the first time you iterate over it. For example, this will print the headline of all entries in the database:

  for e in Entry.objects.all():
      print(e.headline)
  

  Note: Don't use this if all you want to do is determine if at least one result exists. It's more efficient to use exists().

• Asynchronous iteration.. A QuerySet can also be iterated over using async for:

  async for e in Entry.objects.all():
    results.append(e)
  

  Both synchronous and asynchronous iterators of QuerySets share the same underlying cache.

  Changed in Django 4.1:

  Support for asynchronous iteration was added.

• Slicing. As explained in Membatasi QuerySet, a QuerySet can be sliced, using Python's array-slicing syntax. Slicing an unevaluated QuerySet usually returns another unevaluated QuerySet, but Django will execute the database query if you use the "step" parameter of slice syntax, and will return a list. Slicing a QuerySet that has been evaluated also returns a list.

  Also note that even though slicing an unevaluated QuerySet returns another unevaluated QuerySet, modifying it further (e.g., adding more filters, or modifying ordering) is not allowed, since that does not translate well into SQL and it would not have a clear meaning either.

• Pickling/Caching. See the following section for details of what is involved when pickling QuerySets. The important thing for the purposes of this section is that the results are read from the database.

• repr(). A QuerySet is evaluated when you call repr() on it. This is for convenience in the Python interactive interpreter, so you can immediately see your results when using the API interactively.

• len(). A QuerySet is evaluated when you call len() on it. This, as you might expect, returns the length of the result list.

  Note: If you only need to determine the number of records in the set (and don't need the actual objects), it's much more efficient to handle a count at the database level using SQL's SELECT COUNT(*). Django provides a count() method for precisely this reason.

• list(). Force evaluation of a QuerySet by calling list() on it. For example:

  entry_list = list(Entry.objects.all())
  

• bool(). Testing a QuerySet in a boolean context, such as using bool(), or, and or an if statement, will cause the query to be executed. If there is at least one result, the QuerySet is True, otherwise False. For example:

  if Entry.objects.filter(headline="Test"):
     print("There is at least one Entry with the headline Test")
  

  Note: If you only want to determine if at least one result exists (and don't need the actual objects), it's more efficient to use exists().

Pickling QuerySets

If you pickle a QuerySet, this will force all the results to be loaded into memory prior to pickling. Pickling is usually used as a precursor to caching and when the cached queryset is reloaded, you want the results to already be present and ready for use (reading from the database can take some time, defeating the purpose of caching). This means that when you unpickle a QuerySet, it contains the results at the moment it was pickled, rather than the results that are currently in the database.

If you only want to pickle the necessary information to recreate the QuerySet from the database at a later time, pickle the query attribute of the QuerySet. You can then recreate the original QuerySet (without any results loaded) using some code like this:

>>> import pickle
>>> query = pickle.loads(s)     # Assuming 's' is the pickled string.
>>> qs = MyModel.objects.all()
>>> qs.query = query            # Restore the original 'query'.

The query attribute is an opaque object. It represents the internals of the query construction and is not part of the public API. However, it is safe (and fully supported) to pickle and unpickle the attribute's contents as described here.

Restrictions on QuerySet.values_list()

If you recreate QuerySet.values_list() using the pickled query attribute, it will be converted to QuerySet.values():

>>> import pickle
>>> qs = Blog.objects.values_list('id', 'name')
>>> qs
<QuerySet [(1, 'Beatles Blog')]>
>>> reloaded_qs = Blog.objects.all()
>>> reloaded_qs.query = pickle.loads(pickle.dumps(qs.query))
>>> reloaded_qs
<QuerySet [{'id': 1, 'name': 'Beatles Blog'}]>

You can't share pickles between versions

Pickles of QuerySets are only valid for the version of Django that was used to generate them. If you generate a pickle using Django version N, there is no guarantee that pickle will be readable with Django version N+1. Pickles should not be used as part of a long-term archival strategy.

Since pickle compatibility errors can be difficult to diagnose, such as silently corrupted objects, a RuntimeWarning is raised when you try to unpickle a queryset in a Django version that is different than the one in which it was pickled.

API QuerySet

Here's the formal declaration of a QuerySet:

class QuerySet(model=None, query=None, using=None, hints=None)

Usually when you'll interact with a QuerySet you'll use it by chaining filters. To make this work, most QuerySet methods return new querysets. These methods are covered in detail later in this section.

The QuerySet class has the following public attributes you can use for introspection:

ordered

True if the QuerySet is ordered — i.e. has an order_by() clause or a default ordering on the model. False otherwise.

db

Basisdata yang akan digunakan jika permintaan ini dijalankan sekarang.

Catatan

The query parameter to QuerySet exists so that specialized query subclasses can reconstruct internal query state. The value of the parameter is an opaque representation of that query state and is not part of a public API.

Metode yang mengembalikan QuerySet baru

Django provides a range of QuerySet refinement methods that modify either the types of results returned by the QuerySet or the way its SQL query is executed.

Catatan

These methods do not run database queries, therefore they are safe to run in asynchronous code, and do not have separate asynchronous versions.

filter()

filter(*args, **kwargs)

Returns a new QuerySet containing objects that match the given lookup parameters.

The lookup parameters (**kwargs) should be in the format described in Field lookups below. Multiple parameters are joined via AND in the underlying SQL statement.

If you need to execute more complex queries (for example, queries with OR statements), you can use Q objects (*args).

exclude()

exclude(*args, **kwargs)

Returns a new QuerySet containing objects that do not match the given lookup parameters.

The lookup parameters (**kwargs) should be in the format described in Field lookups below. Multiple parameters are joined via AND in the underlying SQL statement, and the whole thing is enclosed in a NOT().

This example excludes all entries whose pub_date is later than 2005-1-3 AND whose headline is "Hello":

Entry.objects.exclude(pub_date__gt=datetime.date(2005, 1, 3), headline='Hello')

In SQL terms, that evaluates to:

SELECT ...
WHERE NOT (pub_date > '2005-1-3' AND headline = 'Hello')

This example excludes all entries whose pub_date is later than 2005-1-3 OR whose headline is "Hello":

Entry.objects.exclude(pub_date__gt=datetime.date(2005, 1, 3)).exclude(headline='Hello')

In SQL terms, that evaluates to:

SELECT ...
WHERE NOT pub_date > '2005-1-3'
AND NOT headline = 'Hello'

Catat bahwa contoh kedua lebih bersifat membatasi.

If you need to execute more complex queries (for example, queries with OR statements), you can use Q objects (*args).

annotate()

annotate(*args, **kwargs)

Annotates each object in the QuerySet with the provided list of query expressions. An expression may be a simple value, a reference to a field on the model (or any related models), or an aggregate expression (averages, sums, etc.) that has been computed over the objects that are related to the objects in the QuerySet.

Each argument to annotate() is an annotation that will be added to each object in the QuerySet that is returned.

The aggregation functions that are provided by Django are described in Aggregation Functions below.

Annotations specified using keyword arguments will use the keyword as the alias for the annotation. Anonymous arguments will have an alias generated for them based upon the name of the aggregate function and the model field that is being aggregated. Only aggregate expressions that reference a single field can be anonymous arguments. Everything else must be a keyword argument.

For example, if you were manipulating a list of blogs, you may want to determine how many entries have been made in each blog:

>>> from django.db.models import Count
>>> q = Blog.objects.annotate(Count('entry'))
# The name of the first blog
>>> q[0].name
'Blogasaurus'
# The number of entries on the first blog
>>> q[0].entry__count
42

The Blog model doesn't define an entry__count attribute by itself, but by using a keyword argument to specify the aggregate function, you can control the name of the annotation:

>>> q = Blog.objects.annotate(number_of_entries=Count('entry'))
# The number of entries on the first blog, using the name provided
>>> q[0].number_of_entries
42

For an in-depth discussion of aggregation, see the topic guide on Aggregation.

alias()

alias(*args, **kwargs)

Same as annotate(), but instead of annotating objects in the QuerySet, saves the expression for later reuse with other QuerySet methods. This is useful when the result of the expression itself is not needed but it is used for filtering, ordering, or as a part of a complex expression. Not selecting the unused value removes redundant work from the database which should result in better performance.

For example, if you want to find blogs with more than 5 entries, but are not interested in the exact number of entries, you could do this:

>>> from django.db.models import Count
>>> blogs = Blog.objects.alias(entries=Count('entry')).filter(entries__gt=5)

alias() can be used in conjunction with annotate(), exclude(), filter(), order_by(), and update(). To use aliased expression with other methods (e.g. aggregate()), you must promote it to an annotation:

Blog.objects.alias(entries=Count('entry')).annotate(
    entries=F('entries'),
).aggregate(Sum('entries'))

filter() and order_by() can take expressions directly, but expression construction and usage often does not happen in the same place (for example, QuerySet method creates expressions, for later use in views). alias() allows building complex expressions incrementally, possibly spanning multiple methods and modules, refer to the expression parts by their aliases and only use annotate() for the final result.

order_by()

order_by(*fields)

By default, results returned by a QuerySet are ordered by the ordering tuple given by the ordering option in the model's Meta. You can override this on a per-QuerySet basis by using the order_by method.

Contoh:

Entry.objects.filter(pub_date__year=2005).order_by('-pub_date', 'headline')

The result above will be ordered by pub_date descending, then by headline ascending. The negative sign in front of "-pub_date" indicates descending order. Ascending order is implied. To order randomly, use "?", like so:

Entry.objects.order_by('?')

Catatan: permintaan order_by('?') mungkin mahal dan lambat, bergantung pada backend basisdata anda sedang gunakan.

Untuk mengurutkan berdasarkan sebuah bidang dalam sebuah model berbeda, gunakan sintaksis sama ketika anda sedang meminta lintas hubungan model. Yaitu, nama dari bidang, diikuti oleh garis bawah ganda (__), diikuti oleh nama dalam model baru, dan seterusnya untuk sebanyak model anda ingin gabungkan. Sebagai contoh:

Entry.objects.order_by('blog__name', 'headline')

Jika anda mencoba mengurutkan berdasarkan sebuah bidang yang hubungan ke model lain, Django akan menggunakan pengurutan awalan pada model terkait, atau diurutkan berdasarkan primary key model terkait jika tidak ada Meta.ordering ditentukan. Sebagai contoh, sejak model Blog tidak mempunyau pengurutan awalan ditentukan:

Entry.objects.order_by('blog')

... mirip ke:

Entry.objects.order_by('blog__id')

Jika Blog mempunyai ordering = ['name'], kemudian himpunan permintaan pertama akan mirip pada:

Entry.objects.order_by('blog__name')

You can also order by query expressions by calling asc() or desc() on the expression:

Entry.objects.order_by(Coalesce('summary', 'headline').desc())

asc() dan desc() mempunyai argumen (nulls_first dan nulls_last) yang mengendalikan bagimana nilai-nilai null diurutkan.

Waspadalah ketika mengurutkan berdasarkan bidang-bidang dalam model terkait jika anda juga menggunakan distinct(). Lihat catatan dalam distinct() untuk sebuah penjelasan dari bagaimana pengurutan model terkait dapat merubah hasil diharapkan.

Catatan

Itu diizinkan untuk menentukan sebuah bidang banyak-nilai untuk mengurutkan hasil dengan (sebagai contoh, sebuah bidang ManyToManyField, atau hubungan balikan dari sebuah bidang ForeignKey).

Pertimbangkan kasus ini:

class Event(Model):
   parent = models.ForeignKey(
       'self',
       on_delete=models.CASCADE,
       related_name='children',
   )
   date = models.DateField()

Event.objects.order_by('children__date')

Here, there could potentially be multiple ordering data for each Event; each Event with multiple children will be returned multiple times into the new QuerySet that order_by() creates. In other words, using order_by() on the QuerySet could return more items than you were working on to begin with - which is probably neither expected nor useful.

Thus, take care when using multi-valued field to order the results. If you can be sure that there will only be one ordering piece of data for each of the items you're ordering, this approach should not present problems. If not, make sure the results are what you expect.

Tidak ada cara menentukan apakah pengurutan harus kasus peka. Sehubungan dengan kasus-kepekaan, Django akan mengurutkan hasil bagaimanapun backend basisdata anda biasanya mengurutkan mereka.

Anda dapat mengurutkan berdasarkan sebuah bidang dirubah menjadi huruf kecil dengan Lower yang akan mencapai pengurutan kasus-tetap:

Entry.objects.order_by(Lower('headline').desc())

Jika anda tidak ingin pengurutan apapun diberlakukan pada permintaan, bahkan tidak pada pengurutan awalan, panggil order_by() tanpa parameter.

You can tell if a query is ordered or not by checking the QuerySet.ordered attribute, which will be True if the QuerySet has been ordered in any way.

Setiap panggilan order_by() akan membersihkan urutan sebelumnya. Sebagai contoh, permintaan ini akan diurutkan berdasarkan pub_date dan bukan headline:

Entry.objects.order_by('headline').order_by('pub_date')

Peringatan

Ordering is not a free operation. Each field you add to the ordering incurs a cost to your database. Each foreign key you add will implicitly include all of its default orderings as well.

If a query doesn't have an ordering specified, results are returned from the database in an unspecified order. A particular ordering is guaranteed only when ordering by a set of fields that uniquely identify each object in the results. For example, if a name field isn't unique, ordering by it won't guarantee objects with the same name always appear in the same order.

reverse()

reverse()

Use the reverse() method to reverse the order in which a queryset's elements are returned. Calling reverse() a second time restores the ordering back to the normal direction.

Untuk mengambil lima abrang "terakhir" dalam himpunan permintaan, anda dapat melakukan ini:

my_queryset.reverse()[:5]

Note that this is not quite the same as slicing from the end of a sequence in Python. The above example will return the last item first, then the penultimate item and so on. If we had a Python sequence and looked at seq[-5:], we would see the fifth-last item first. Django doesn't support that mode of access (slicing from the end), because it's not possible to do it efficiently in SQL.

Also, note that reverse() should generally only be called on a QuerySet which has a defined ordering (e.g., when querying against a model which defines a default ordering, or when using order_by()). If no such ordering is defined for a given QuerySet, calling reverse() on it has no real effect (the ordering was undefined prior to calling reverse(), and will remain undefined afterward).

distinct()

distinct(*fields)

Returns a new QuerySet that uses SELECT DISTINCT in its SQL query. This eliminates duplicate rows from the query results.

By default, a QuerySet will not eliminate duplicate rows. In practice, this is rarely a problem, because simple queries such as Blog.objects.all() don't introduce the possibility of duplicate result rows. However, if your query spans multiple tables, it's possible to get duplicate results when a QuerySet is evaluated. That's when you'd use distinct().

Catatan

Any fields used in an order_by() call are included in the SQL SELECT columns. This can sometimes lead to unexpected results when used in conjunction with distinct(). If you order by fields from a related model, those fields will be added to the selected columns and they may make otherwise duplicate rows appear to be distinct. Since the extra columns don't appear in the returned results (they are only there to support ordering), it sometimes looks like non-distinct results are being returned.

Similarly, if you use a values() query to restrict the columns selected, the columns used in any order_by() (or default model ordering) will still be involved and may affect uniqueness of the results.

The moral here is that if you are using distinct() be careful about ordering by related models. Similarly, when using distinct() and values() together, be careful when ordering by fields not in the values() call.

On PostgreSQL only, you can pass positional arguments (*fields) in order to specify the names of fields to which the DISTINCT should apply. This translates to a SELECT DISTINCT ON SQL query. Here's the difference. For a normal distinct() call, the database compares each field in each row when determining which rows are distinct. For a distinct() call with specified field names, the database will only compare the specified field names.

Catatan

When you specify field names, you must provide an order_by() in the QuerySet, and the fields in order_by() must start with the fields in distinct(), in the same order.

For example, SELECT DISTINCT ON (a) gives you the first row for each value in column a. If you don't specify an order, you'll get some arbitrary row.

Contoh-contoh (setelah itu pertama hanya akan bekerja pada PostgreSQL):

>>> Author.objects.distinct()
[...]

>>> Entry.objects.order_by('pub_date').distinct('pub_date')
[...]

>>> Entry.objects.order_by('blog').distinct('blog')
[...]

>>> Entry.objects.order_by('author', 'pub_date').distinct('author', 'pub_date')
[...]

>>> Entry.objects.order_by('blog__name', 'mod_date').distinct('blog__name', 'mod_date')
[...]

>>> Entry.objects.order_by('author', 'pub_date').distinct('author')
[...]

Catatan

Keep in mind that order_by() uses any default related model ordering that has been defined. You might have to explicitly order by the relation _id or referenced field to make sure the DISTINCT ON expressions match those at the beginning of the ORDER BY clause. For example, if the Blog model defined an ordering by name:

Entry.objects.order_by('blog').distinct('blog')

...wouldn't work because the query would be ordered by blog__name thus mismatching the DISTINCT ON expression. You'd have to explicitly order by the relation _id field (blog_id in this case) or the referenced one (blog__pk) to make sure both expressions match.

values()

values(*fields, **expressions)

Returns a QuerySet that returns dictionaries, rather than model instances, when used as an iterable.

Each of those dictionaries represents an object, with the keys corresponding to the attribute names of model objects.

Contoh ini membandingkan kamus-kamus dari values() dengan obyek model biasa:

# This list contains a Blog object.
>>> Blog.objects.filter(name__startswith='Beatles')
<QuerySet [<Blog: Beatles Blog>]>

# This list contains a dictionary.
>>> Blog.objects.filter(name__startswith='Beatles').values()
<QuerySet [{'id': 1, 'name': 'Beatles Blog', 'tagline': 'All the latest Beatles news.'}]>

The values() method takes optional positional arguments, *fields, which specify field names to which the SELECT should be limited. If you specify the fields, each dictionary will contain only the field keys/values for the fields you specify. If you don't specify the fields, each dictionary will contain a key and value for every field in the database table.

Contoh:

>>> Blog.objects.values()
<QuerySet [{'id': 1, 'name': 'Beatles Blog', 'tagline': 'All the latest Beatles news.'}]>
>>> Blog.objects.values('id', 'name')
<QuerySet [{'id': 1, 'name': 'Beatles Blog'}]>

The values() method also takes optional keyword arguments, **expressions, which are passed through to annotate():

>>> from django.db.models.functions import Lower
>>> Blog.objects.values(lower_name=Lower('name'))
<QuerySet [{'lower_name': 'beatles blog'}]>

You can use built-in and custom lookups in ordering. For example:

>>> from django.db.models import CharField
>>> from django.db.models.functions import Lower
>>> CharField.register_lookup(Lower)
>>> Blog.objects.values('name__lower')
<QuerySet [{'name__lower': 'beatles blog'}]>

An aggregate within a values() clause is applied before other arguments within the same values() clause. If you need to group by another value, add it to an earlier values() clause instead. For example:

>>> from django.db.models import Count
>>> Blog.objects.values('entry__authors', entries=Count('entry'))
<QuerySet [{'entry__authors': 1, 'entries': 20}, {'entry__authors': 1, 'entries': 13}]>
>>> Blog.objects.values('entry__authors').annotate(entries=Count('entry'))
<QuerySet [{'entry__authors': 1, 'entries': 33}]>

Sedikit kebijakan yang berharga disebutkan:

• If you have a field called foo that is a ForeignKey, the default values() call will return a dictionary key called foo_id, since this is the name of the hidden model attribute that stores the actual value (the foo attribute refers to the related model). When you are calling values() and passing in field names, you can pass in either foo or foo_id and you will get back the same thing (the dictionary key will match the field name you passed in).

  Sebagai contoh:

  >>> Entry.objects.values()
  <QuerySet [{'blog_id': 1, 'headline': 'First Entry', ...}, ...]>
  
  >>> Entry.objects.values('blog')
  <QuerySet [{'blog': 1}, ...]>
  
  >>> Entry.objects.values('blog_id')
  <QuerySet [{'blog_id': 1}, ...]>
  

• When using values() together with distinct(), be aware that ordering can affect the results. See the note in distinct() for details.

• If you use a values() clause after an extra() call, any fields defined by a select argument in the extra() must be explicitly included in the values() call. Any extra() call made after a values() call will have its extra selected fields ignored.

• Calling only() and defer() after values() doesn't make sense, so doing so will raise a TypeError.

• Combining transforms and aggregates requires the use of two annotate() calls, either explicitly or as keyword arguments to values(). As above, if the transform has been registered on the relevant field type the first annotate() can be omitted, thus the following examples are equivalent:

  >>> from django.db.models import CharField, Count
  >>> from django.db.models.functions import Lower
  >>> CharField.register_lookup(Lower)
  >>> Blog.objects.values('entry__authors__name__lower').annotate(entries=Count('entry'))
  <QuerySet [{'entry__authors__name__lower': 'test author', 'entries': 33}]>
  >>> Blog.objects.values(
  ...     entry__authors__name__lower=Lower('entry__authors__name')
  ... ).annotate(entries=Count('entry'))
  <QuerySet [{'entry__authors__name__lower': 'test author', 'entries': 33}]>
  >>> Blog.objects.annotate(
  ...     entry__authors__name__lower=Lower('entry__authors__name')
  ... ).values('entry__authors__name__lower').annotate(entries=Count('entry'))
  <QuerySet [{'entry__authors__name__lower': 'test author', 'entries': 33}]>
  

It is useful when you know you're only going to need values from a small number of the available fields and you won't need the functionality of a model instance object. It's more efficient to select only the fields you need to use.

Finally, note that you can call filter(), order_by(), etc. after the values() call, that means that these two calls are identical:

Blog.objects.values().order_by('id')
Blog.objects.order_by('id').values()

The people who made Django prefer to put all the SQL-affecting methods first, followed (optionally) by any output-affecting methods (such as values()), but it doesn't really matter. This is your chance to really flaunt your individualism.

You can also refer to fields on related models with reverse relations through OneToOneField, ForeignKey and ManyToManyField attributes:

>>> Blog.objects.values('name', 'entry__headline')
<QuerySet [{'name': 'My blog', 'entry__headline': 'An entry'},
     {'name': 'My blog', 'entry__headline': 'Another entry'}, ...]>

Peringatan

Because ManyToManyField attributes and reverse relations can have multiple related rows, including these can have a multiplier effect on the size of your result set. This will be especially pronounced if you include multiple such fields in your values() query, in which case all possible combinations will be returned.

Special values for JSONField on SQLite

Due to the way the JSON_EXTRACT and JSON_TYPE SQL functions are implemented on SQLite, and lack of the BOOLEAN data type, values() will return True, False, and None instead of "true", "false", and "null" strings for JSONField key transforms.

values_list()

values_list(*fields, flat=False, named=False)

This is similar to values() except that instead of returning dictionaries, it returns tuples when iterated over. Each tuple contains the value from the respective field or expression passed into the values_list() call — so the first item is the first field, etc. For example:

>>> Entry.objects.values_list('id', 'headline')
<QuerySet [(1, 'First entry'), ...]>
>>> from django.db.models.functions import Lower
>>> Entry.objects.values_list('id', Lower('headline'))
<QuerySet [(1, 'first entry'), ...]>

If you only pass in a single field, you can also pass in the flat parameter. If True, this will mean the returned results are single values, rather than one-tuples. An example should make the difference clearer:

>>> Entry.objects.values_list('id').order_by('id')
<QuerySet[(1,), (2,), (3,), ...]>

>>> Entry.objects.values_list('id', flat=True).order_by('id')
<QuerySet [1, 2, 3, ...]>

Itu adalah sebuah kesalahan melewatkan dalam flat ketika ada lebih dari satu bidang.

You can pass named=True to get results as a namedtuple():

>>> Entry.objects.values_list('id', 'headline', named=True)
<QuerySet [Row(id=1, headline='First entry'), ...]>

Using a named tuple may make use of the results more readable, at the expense of a small performance penalty for transforming the results into a named tuple.

Jika anda tidak melewatkan nilai apapun ke values_list(), itu akan mengembalikan semua bidang dalam model, dalam urutan mereka dinyatakan.

A common need is to get a specific field value of a certain model instance. To achieve that, use values_list() followed by a get() call:

>>> Entry.objects.values_list('headline', flat=True).get(pk=1)
'First entry'

values() and values_list() are both intended as optimizations for a specific use case: retrieving a subset of data without the overhead of creating a model instance. This metaphor falls apart when dealing with many-to-many and other multivalued relations (such as the one-to-many relation of a reverse foreign key) because the "one row, one object" assumption doesn't hold.

For example, notice the behavior when querying across a ManyToManyField:

>>> Author.objects.values_list('name', 'entry__headline')
<QuerySet [('Noam Chomsky', 'Impressions of Gaza'),
 ('George Orwell', 'Why Socialists Do Not Believe in Fun'),
 ('George Orwell', 'In Defence of English Cooking'),
 ('Don Quixote', None)]>

Authors with multiple entries appear multiple times and authors without any entries have None for the entry headline.

Similarly, when querying a reverse foreign key, None appears for entries not having any author:

>>> Entry.objects.values_list('authors')
<QuerySet [('Noam Chomsky',), ('George Orwell',), (None,)]>

Special values for JSONField on SQLite

Due to the way the JSON_EXTRACT and JSON_TYPE SQL functions are implemented on SQLite, and lack of the BOOLEAN data type, values_list() will return True, False, and None instead of "true", "false", and "null" strings for JSONField key transforms.

dates()

dates(field, kind, order='ASC')

Returns a QuerySet that evaluates to a list of datetime.date objects representing all available dates of a particular kind within the contents of the QuerySet.

field should be the name of a DateField of your model. kind should be either "year", "month", "week", or "day". Each datetime.date object in the result list is "truncated" to the given type.

• "year" returns a list of all distinct year values for the field.
• "month" returns a list of all distinct year/month values for the field.
• "week" returns a list of all distinct year/week values for the field. All dates will be a Monday.
• "day" returns a list of all distinct year/month/day values for the field.

order, which defaults to 'ASC', should be either 'ASC' or 'DESC'. This specifies how to order the results.

Contoh:

>>> Entry.objects.dates('pub_date', 'year')
[datetime.date(2005, 1, 1)]
>>> Entry.objects.dates('pub_date', 'month')
[datetime.date(2005, 2, 1), datetime.date(2005, 3, 1)]
>>> Entry.objects.dates('pub_date', 'week')
[datetime.date(2005, 2, 14), datetime.date(2005, 3, 14)]
>>> Entry.objects.dates('pub_date', 'day')
[datetime.date(2005, 2, 20), datetime.date(2005, 3, 20)]
>>> Entry.objects.dates('pub_date', 'day', order='DESC')
[datetime.date(2005, 3, 20), datetime.date(2005, 2, 20)]
>>> Entry.objects.filter(headline__contains='Lennon').dates('pub_date', 'day')
[datetime.date(2005, 3, 20)]

datetimes()

datetimes(field_name, kind, order='ASC', tzinfo=None, is_dst=None)

Returns a QuerySet that evaluates to a list of datetime.datetime objects representing all available dates of a particular kind within the contents of the QuerySet.

field_name harus berupa nama dari DateTimeField model anda.

kind should be either "year", "month", "week", "day", "hour", "minute", or "second". Each datetime.datetime object in the result list is "truncated" to the given type.

order, which defaults to 'ASC', should be either 'ASC' or 'DESC'. This specifies how to order the results.

tzinfo defines the time zone to which datetimes are converted prior to truncation. Indeed, a given datetime has different representations depending on the time zone in use. This parameter must be a datetime.tzinfo object. If it's None, Django uses the current time zone. It has no effect when USE_TZ is False.

is_dst indicates whether or not pytz should interpret nonexistent and ambiguous datetimes in daylight saving time. By default (when is_dst=None), pytz raises an exception for such datetimes.

Ditinggalkan sejak versi 4.0: The is_dst parameter is deprecated and will be removed in Django 5.0.

Catatan

This function performs time zone conversions directly in the database. As a consequence, your database must be able to interpret the value of tzinfo.tzname(None). This translates into the following requirements:

• SQLite: no requirements. Conversions are performed in Python.
• PostgreSQL: tidak ada persyaratan (lihat Time Zones).
• Oracle: tidak ada persyaratan (lihat Choosing a Time Zone File).
• MySQL: memuat tabel zona waktu dengan mysql_tzinfo_to_sql.

none()

none()

Calling none() will create a queryset that never returns any objects and no query will be executed when accessing the results. A qs.none() queryset is an instance of EmptyQuerySet.

Contoh:

>>> Entry.objects.none()
<QuerySet []>
>>> from django.db.models.query import EmptyQuerySet
>>> isinstance(Entry.objects.none(), EmptyQuerySet)
True

all()

all()

Returns a copy of the current QuerySet (or QuerySet subclass). This can be useful in situations where you might want to pass in either a model manager or a QuerySet and do further filtering on the result. After calling all() on either object, you'll definitely have a QuerySet to work with.

When a QuerySet is evaluated, it typically caches its results. If the data in the database might have changed since a QuerySet was evaluated, you can get updated results for the same query by calling all() on a previously evaluated QuerySet.

union()

union(*other_qs, all=False)

Uses SQL's UNION operator to combine the results of two or more QuerySets. For example:

>>> qs1.union(qs2, qs3)

The UNION operator selects only distinct values by default. To allow duplicate values, use the all=True argument.

union(), intersection(), and difference() return model instances of the type of the first QuerySet even if the arguments are QuerySets of other models. Passing different models works as long as the SELECT list is the same in all QuerySets (at least the types, the names don't matter as long as the types are in the same order). In such cases, you must use the column names from the first QuerySet in QuerySet methods applied to the resulting QuerySet. For example:

>>> qs1 = Author.objects.values_list('name')
>>> qs2 = Entry.objects.values_list('headline')
>>> qs1.union(qs2).order_by('name')

In addition, only LIMIT, OFFSET, COUNT(*), ORDER BY, and specifying columns (i.e. slicing, count(), exists(), order_by(), and values()/values_list()) are allowed on the resulting QuerySet. Further, databases place restrictions on what operations are allowed in the combined queries. For example, most databases don't allow LIMIT or OFFSET in the combined queries.

intersection()

intersection(*other_qs)

Uses SQL's INTERSECT operator to return the shared elements of two or more QuerySets. For example:

>>> qs1.intersection(qs2, qs3)

Lihat union() untuk beberapa pembatasan.

difference()

difference(*other_qs)

Uses SQL's EXCEPT operator to keep only elements present in the QuerySet but not in some other QuerySets. For example:

>>> qs1.difference(qs2, qs3)

Lihat union() untuk beberapa pembatasan.

select_related()

select_related(*fields)

Returns a QuerySet that will "follow" foreign-key relationships, selecting additional related-object data when it executes its query. This is a performance booster which results in a single more complex query but means later use of foreign-key relationships won't require database queries.

The following examples illustrate the difference between plain lookups and select_related() lookups. Here's standard lookup:

# Hits the database.
e = Entry.objects.get(id=5)

# Hits the database again to get the related Blog object.
b = e.blog

Dan ini adalah pencarian select_related:

# Hits the database.
e = Entry.objects.select_related('blog').get(id=5)

# Doesn't hit the database, because e.blog has been prepopulated
# in the previous query.
b = e.blog

Anda dapat menggunakan select_related() dengan himpunan permintaan apapun dari obyek:

from django.utils import timezone

# Find all the blogs with entries scheduled to be published in the future.
blogs = set()

for e in Entry.objects.filter(pub_date__gt=timezone.now()).select_related('blog'):
    # Without select_related(), this would make a database query for each
    # loop iteration in order to fetch the related blog for each entry.
    blogs.add(e.blog)

urutan dari rantaian filter() dan select_related() tidak penting. Himpunan permintaan ini adalah setara:

Entry.objects.filter(pub_date__gt=timezone.now()).select_related('blog')
Entry.objects.select_related('blog').filter(pub_date__gt=timezone.now())

Dan anda dapat mengikuti foreign key dalam cara yang mirip untuk meminta mereka. Jika anda memiliki model-model berikut:

from django.db import models

class City(models.Model):
    # ...
    pass

class Person(models.Model):
    # ...
    hometown = models.ForeignKey(
        City,
        on_delete=models.SET_NULL,
        blank=True,
        null=True,
    )

class Book(models.Model):
    # ...
    author = models.ForeignKey(Person, on_delete=models.CASCADE)

... kemudian sebuah panggilan pada Book.objects.select_related('author__hometown').get(id=4) akan menyimpan sementara Person terkait dan City terkait:

# Hits the database with joins to the author and hometown tables.
b = Book.objects.select_related('author__hometown').get(id=4)
p = b.author         # Doesn't hit the database.
c = p.hometown       # Doesn't hit the database.

# Without select_related()...
b = Book.objects.get(id=4)  # Hits the database.
p = b.author         # Hits the database.
c = p.hometown       # Hits the database.

Anda mengacu pada ForeignKey apapun atau hubungan OneToOneField dalam daftar dari bidang-bidang dilewatkan ke select_related().

You can also refer to the reverse direction of a OneToOneField in the list of fields passed to select_related — that is, you can traverse a OneToOneField back to the object on which the field is defined. Instead of specifying the field name, use the related_name for the field on the related object.

There may be some situations where you wish to call select_related() with a lot of related objects, or where you don't know all of the relations. In these cases it is possible to call select_related() with no arguments. This will follow all non-null foreign keys it can find - nullable foreign keys must be specified. This is not recommended in most cases as it is likely to make the underlying query more complex, and return more data, than is actually needed.

If you need to clear the list of related fields added by past calls of select_related on a QuerySet, you can pass None as a parameter:

>>> without_relations = queryset.select_related(None)

Chaining select_related calls works in a similar way to other methods - that is that select_related('foo', 'bar') is equivalent to select_related('foo').select_related('bar').

prefetch_related()

prefetch_related(*lookups)

Returns a QuerySet that will automatically retrieve, in a single batch, related objects for each of the specified lookups.

This has a similar purpose to select_related, in that both are designed to stop the deluge of database queries that is caused by accessing related objects, but the strategy is quite different.

select_related works by creating an SQL join and including the fields of the related object in the SELECT statement. For this reason, select_related gets the related objects in the same database query. However, to avoid the much larger result set that would result from joining across a 'many' relationship, select_related is limited to single-valued relationships - foreign key and one-to-one.

prefetch_related, di sisi lain, melakukan pencarian terpisah untuk setiap hubungan, dan melakukan 'joining' di Python. Ini mengizinkan itu untuk mengambil obyek many-to-one, yang tidak dapat diselesaikan menggunakan select_related, sebagai tambahan pada foreign key dan hubungan one-to-one yang didukung oleh select_related. Itu juga mendukung pengambilan dari GenericRelation dan GenericForeignKey, bagaimanapun, itu harus dibatasi pada kumpulan sama jenis dari hasil. Sebagai contoh, mengambil obyek diacukan oleh GenericForeignKey hanya didukung jika permintaan dibatasi pada satu ContentType.

Sebagai contoh, andaikan anda mempunyai dua model ini:

from django.db import models

class Topping(models.Model):
    name = models.CharField(max_length=30)

class Pizza(models.Model):
    name = models.CharField(max_length=50)
    toppings = models.ManyToManyField(Topping)

    def __str__(self):
        return "%s (%s)" % (
            self.name,
            ", ".join(topping.name for topping in self.toppings.all()),
        )

dan jalankan:

>>> Pizza.objects.all()
["Hawaiian (ham, pineapple)", "Seafood (prawns, smoked salmon)"...

Masalah dengan ini adalah bahwa setiap kali Pizza.__str__() bertanya untuk self.toppings.all() itu harus meminta basisdata, jadi Pizza.objects.all() akan menjalankan sebuah permintaan pada tabel Toppings untuk setiap barang dalam QuerySet Pizza.

Kami dapat mengurangi hanya dua permintaan menggunakan prefetch_related:

>>> Pizza.objects.prefetch_related('toppings')

Ini mengartikan sebuah self.toppings.all() untuk setiap Pizza, sekarang setiap kali self.toppings.all() dipanggil, daripada harus pergi ke basisdata untuk barang-barang, itu akan menemukan mereka dalam pra pengambilan cache QuerySet yang dikumpulkan dalam sebuah permintaan tunggal.

Yaitu, semua taburan berhubungan akan diambil dalam sebuah permintaan tunggal, dan digunakan untuk membuat QuerySets yang mempunyai cache pra-diisi dari hasil terkait; QuerySet ini kemudian digunakan dalam panggilan self.toppings.all().

Permintaan-permintaan tambahan dalam prefetch_related() dijalankan setelah QuerySet telah mulai dinilai dan permintaan utama telah dijalankan.

Jika anda mempunyai sebuah perulangan dari instance model, anda dapat mengambil atribut terkait pada instance tersebut menggunakan fungsi prefetch_related_objects().

Note that the result cache of the primary QuerySet and all specified related objects will then be fully loaded into memory. This changes the typical behavior of QuerySets, which normally try to avoid loading all objects into memory before they are needed, even after a query has been executed in the database.

Catatan

Remember that, as always with QuerySets, any subsequent chained methods which imply a different database query will ignore previously cached results, and retrieve data using a fresh database query. So, if you write the following:

>>> pizzas = Pizza.objects.prefetch_related('toppings')
>>> [list(pizza.toppings.filter(spicy=True)) for pizza in pizzas]

...kemudian fakta bahwa pizza.toppings.all() telah di pengambilan tidak akan membantu anda. prefetch_related('toppings') menyisipkan pizza.toppings.all(), tetapi pizza.toppings.filter() adalah sebuah baru dan permintaan berbeda. Cache pra pengambilan tidak dapat membantu disini; faktanya itu melukai penampilan, sejak anda telah selesai sebuah permintaan basisdata yang anda belum gunakan. Jadi gunakan fitur ini dengan waspada!

Juga, jika anda memanggil metode perubahan-basisdata add(), remove(), clear() atau set(), pada related managers, cache dipindahkan apapun untuk hubungan akan dibersihkan.

You can also use the normal join syntax to do related fields of related fields. Suppose we have an additional model to the example above:

class Restaurant(models.Model):
    pizzas = models.ManyToManyField(Pizza, related_name='restaurants')
    best_pizza = models.ForeignKey(Pizza, related_name='championed_by', on_delete=models.CASCADE)

Berikut ini adalah semua sah:

>>> Restaurant.objects.prefetch_related('pizzas__toppings')

Ini akan mengambil semua pizza milik restoran, dan semua taburan melik ke pizza tersebut. Ini akan menghasilkan sejumlah 3 permintaan basisdata - satu untuk restoran, satu untuk pizza, dan satu untuk taburan.

>>> Restaurant.objects.prefetch_related('best_pizza__toppings')

This will fetch the best pizza and all the toppings for the best pizza for each restaurant. This will be done in 3 database queries - one for the restaurants, one for the 'best pizzas', and one for the toppings.

The best_pizza relationship could also be fetched using select_related to reduce the query count to 2:

>>> Restaurant.objects.select_related('best_pizza').prefetch_related('best_pizza__toppings')

Sejak pengambilan dijalankan setelah permintaan utama (yang menyertakan penggabungan dibutuhkan oleh select_related), itu dapat mengenali bahwa obyek best_pizza telah diambil, dan itu akan melewati pengambilan kembali.

Mengikat panggilan prefetch_related akan mengumpulkan pencarian yang diambil. Untuk membersihkan perilaku prefetch_related apapun, melewatkan None sebagai sebuah parameter:

>>> non_prefetched = qs.prefetch_related(None)

Satu perbedaan untuk dicatat ketika menggunakan prefetch_related adalah bahwa obyek dibuat oleh sebuah permintaan dapat dibagi diantara obyek-obyek berbeda yang mereka terkait pada yaitu instance model Python tunggal dapat muncul lebih dari satu titik dalam pohon dari obyek yang dikembalikan. Ini akan biasanya terjadi dengan hubungan foreign key. Khususnya perilaku ini akan tidak menjadi sebuah masalah, dan akan dalam faktanya menyimpan kedua memori dan waktu CPU.

Selagi prefetch_related mendukung pengambilan hubungan GenericForeignKey, sejumlah permintaan akan bergantung pada data. Sejak sebuah GenericForeignKey dapat mengacu data dalam banyak tabel, satu permiintaan per tabel acuan adalah dibutuhkan, daripada satu permintaan untuk semua barang. Ada mungkin permintaan tambahan pada tabel ContentType jika baris bersangkutan belum siambil.

prefetch_related dalam kebanyakan kasus akan diterapkan menggunakan sebuah permintaan SQL yang menggunakan penghubung 'IN'. Ini berarti bahwa untuk QuerySet besar sebuah klausa 'IN' besar dapat dibangkitkan, yang, bergantung pada basisdata, mungkin memiliki masalah penampilan dari miliknya sendiri ketika itu datang mengurai atau menjalankan permintaan SQL. Selalu profil untuk kasus penggunaan anda!

Changed in Django 4.1:

If you use iterator() to run the query, prefetch_related() calls will only be observed if a value for chunk_size is provided.

Anda dapat menggunakan obyek Prefetch untuk lebih lanjut mengendalikan tindakan pengambilan.

Dalam formulir termudahnya Prefetch setara pada pencarian berdasarkan string tradisional:

>>> from django.db.models import Prefetch
>>> Restaurant.objects.prefetch_related(Prefetch('pizzas__toppings'))

You can provide a custom queryset with the optional queryset argument. This can be used to change the default ordering of the queryset:

>>> Restaurant.objects.prefetch_related(
...     Prefetch('pizzas__toppings', queryset=Toppings.objects.order_by('name')))

Or to call select_related() when applicable to reduce the number of queries even further:

>>> Pizza.objects.prefetch_related(
...     Prefetch('restaurants', queryset=Restaurant.objects.select_related('best_pizza')))

Anda dapat juga memberikan hasil pengambilan pada atribut penyesuaian dengan pilihan argumen to_attr. Hasil akan disimpan langsung dalam sebuah daftar.

Ini mengizinkan mengambil hubungan sama banyak kali dengan QuerySet berbeda; sebagai contoh:

>>> vegetarian_pizzas = Pizza.objects.filter(vegetarian=True)
>>> Restaurant.objects.prefetch_related(
...     Prefetch('pizzas', to_attr='menu'),
...     Prefetch('pizzas', queryset=vegetarian_pizzas, to_attr='vegetarian_menu'))

Lookups created with custom to_attr can still be traversed as usual by other lookups:

>>> vegetarian_pizzas = Pizza.objects.filter(vegetarian=True)
>>> Restaurant.objects.prefetch_related(
...     Prefetch('pizzas', queryset=vegetarian_pizzas, to_attr='vegetarian_menu'),
...     'vegetarian_menu__toppings')

Menggunakan to_attr dianjurkan ketika menyaring turun hasil pengambilan ketika itu sedikit membingungkan daripada menyimpan hasil disaring dalam cache pengelola terkait:

>>> queryset = Pizza.objects.filter(vegetarian=True)
>>>
>>> # Recommended:
>>> restaurants = Restaurant.objects.prefetch_related(
...     Prefetch('pizzas', queryset=queryset, to_attr='vegetarian_pizzas'))
>>> vegetarian_pizzas = restaurants[0].vegetarian_pizzas
>>>
>>> # Not recommended:
>>> restaurants = Restaurant.objects.prefetch_related(
...     Prefetch('pizzas', queryset=queryset))
>>> vegetarian_pizzas = restaurants[0].pizzas.all()

Pengambilan penyesuaian juga bekerja dengan hubungan terkait tunggal seperti menyampaikan ForeignKey atau OneToOneField. Umumnya anda akan ingin menggunakan select_related() untuk hubungan ini, tetapi ada sejumlah kasus dimana pengambilan dengan QuerySet penyesuaian berguna:

• Anda ingin menggunakan sebuah QuerySet yang melakukan pengambilan lebih lanjut pada model terkait.

• Anda ingin hanya mengambil sebah sub kumpulan dari obyek-obyek terkait.

• You want to use performance optimization techniques like deferred fields:

  >>> queryset = Pizza.objects.only('name')
  >>>
  >>> restaurants = Restaurant.objects.prefetch_related(
  ...     Prefetch('best_pizza', queryset=queryset))
  

When using multiple databases, Prefetch will respect your choice of database. If the inner query does not specify a database, it will use the database selected by the outer query. All of the following are valid:

>>> # Both inner and outer queries will use the 'replica' database
>>> Restaurant.objects.prefetch_related('pizzas__toppings').using('replica')
>>> Restaurant.objects.prefetch_related(
...     Prefetch('pizzas__toppings'),
... ).using('replica')
>>>
>>> # Inner will use the 'replica' database; outer will use 'default' database
>>> Restaurant.objects.prefetch_related(
...     Prefetch('pizzas__toppings', queryset=Toppings.objects.using('replica')),
... )
>>>
>>> # Inner will use 'replica' database; outer will use 'cold-storage' database
>>> Restaurant.objects.prefetch_related(
...     Prefetch('pizzas__toppings', queryset=Toppings.objects.using('replica')),
... ).using('cold-storage')

Catatan

Urutan pencarian masalah.

Ambil contoh-contoh berikut:

>>> prefetch_related('pizzas__toppings', 'pizzas')

Ini bekerja meskipun itu adalah tidak berurutan karena 'pizzas__toppings' sudah mengandung semua informasi yang dibutuhkan, oleh karena itu argumen kedua 'pizzas' adalah sebenarnya berulang.

>>> prefetch_related('pizzas__toppings', Prefetch('pizzas', queryset=Pizza.objects.all()))

Ini akan memunculkan sebuah ValueError karena dari usaha untuk menentukan kembali queryset dari sebelumnya dicari. Catat bahwa sebuah queryset tersirat telah dibuat untuk melintasi 'pizzas' sebagai bagian dari pencarian 'pizzas__toppings'.

>>> prefetch_related('pizza_list__toppings', Prefetch('pizzas', to_attr='pizza_list'))

Ini akan memicu sebuah AttributeError karena 'pizza_list' tidak ada ketika 'pizza_list__toppings' sedang diolah.

Pertimbangan ini tidak terbatas untuk digunakan dari obyek Prefetch. Beberapa teknik tingkat lanjut mungkin membutuhkan bahwa pencarian dilakukan dalam perintah khusus untuk menghindari permintaan tambahan; oleh karena itu dianjurkan untuk selalu berhari-hari memesan argumen prefetch_related.

extra()

extra(select=None, where=None, params=None, tables=None, order_by=None, select_params=None)

Sometimes, the Django query syntax by itself can't easily express a complex WHERE clause. For these edge cases, Django provides the extra() QuerySet modifier — a hook for injecting specific clauses into the SQL generated by a QuerySet.

Gunakan metode ini sebagai usaha terakhir

This is an old API that we aim to deprecate at some point in the future. Use it only if you cannot express your query using other queryset methods. If you do need to use it, please file a ticket using the QuerySet.extra keyword with your use case (please check the list of existing tickets first) so that we can enhance the QuerySet API to allow removing extra(). We are no longer improving or fixing bugs for this method.

Sebagai contoh, ini menggunakan extra():

>>> qs.extra(
...     select={'val': "select col from sometable where othercol = %s"},
...     select_params=(someparam,),
... )

setara pada:

>>> qs.annotate(val=RawSQL("select col from sometable where othercol = %s", (someparam,)))

Keuntungan utama dari penggunaan RawSQL adalah bahwa anda dapat menyetel output_field jika dibutuhkan. kerugian utama adalah jika anda mengacu pada beberapa nama lain tabel dari queryset dalam SQL mentah, kemudian itu memungkinkan Django mungkin merubah nama lain itu (sebagai contoh, ketika queryset didunakan sebagai subpermintaan di permintaan lain lagi).

Peringatan

Anda harus sangat berhati-hati kapanpun anda menggunakan extra(). Setiap waktu anda menggunakan itu, anda harus meloloskan parameter apapun dimana pengguna dapat mengendalikan menggunakan params untuk melindungi terhadap serangan suntikan SQL.

You also must not quote placeholders in the SQL string. This example is vulnerable to SQL injection because of the quotes around %s:

SELECT col FROM sometable WHERE othercol = '%s'  # unsafe!

Anda dapat membaca lebih tentang bagaimana SQL injection protection Django bekerja.

Berdasarkan pengertian, pencarian tambahan ini mungkin tidak dapat di hubungkan pada mesin basisdata lain (karena anda secara tegas menulis kode SQL) dan melanggar prinsip DRY, jadi anda harus menghindari mereka jika memungkinkan.

Tentukan satu atau lebih dari params, select, where atau tables. Tidak satupun dari argumen dibutuhkan, tetapi anda harus menggunakan setidaknya satu dari mereka.

• select

  Argumen select membiarkan anda menaruh bidang tambahan dalam klausa SELECT. Itu harus berupa nama-nama atribut pemetaan sebuah dictionary pada SQL untuk digunakan menghitung atribut itu.

  Contoh:

  Entry.objects.extra(select={'is_recent': "pub_date > '2006-01-01'"})
  

  Sebagai hasil, setiap obyek Entry masukan mempunyai satu atribut tambahan, is_recent, sebuah boolean mewakili apakah pub_date masukan lebih besar dari 1 Januari 2006.

  Django inserts the given SQL snippet directly into the SELECT statement, so the resulting SQL of the above example would be something like:

  SELECT blog_entry.*, (pub_date > '2006-01-01') AS is_recent
  FROM blog_entry;
  

  Contoh selanjutnya lebih tinggi; itu melakukan subpermintaan untuk memberikan setiap obyek Blog hasil sebuah atribut entry_count, sebuah hitungan integer dari obyek Entry terkait .

  Blog.objects.extra(
      select={
          'entry_count': 'SELECT COUNT(*) FROM blog_entry WHERE blog_entry.blog_id = blog_blog.id'
      },
  )
  

  In this particular case, we're exploiting the fact that the query will already contain the blog_blog table in its FROM clause.

  The resulting SQL of the above example would be:

  SELECT blog_blog.*, (SELECT COUNT(*) FROM blog_entry WHERE blog_entry.blog_id = blog_blog.id) AS entry_count
  FROM blog_blog;
  

  Note that the parentheses required by most database engines around subqueries are not required in Django's select clauses. Also note that some database backends, such as some MySQL versions, don't support subqueries.

  In some rare cases, you might wish to pass parameters to the SQL fragments in extra(select=...). For this purpose, use the select_params parameter.

  Ini akan bekerja, sebagai contoh:

  Blog.objects.extra(
      select={'a': '%s', 'b': '%s'},
      select_params=('one', 'two'),
  )
  

  Jika anda butuh menggunakan sebuah harfiah %s didalam string terpilih anda, gunakan urutan %%s.

• where / tables

  You can define explicit SQL WHERE clauses — perhaps to perform non-explicit joins — by using where. You can manually add tables to the SQL FROM clause by using tables.

  where and tables both take a list of strings. All where parameters are "AND"ed to any other search criteria.

  Contoh:

  Entry.objects.extra(where=["foo='a' OR bar = 'a'", "baz = 'a'"])
  

  ...translates (roughly) into the following SQL:

  SELECT * FROM blog_entry WHERE (foo='a' OR bar='a') AND (baz='a')
  

  Berhati-hatilan ketika menggunakan parameter tables jika anda sedang menentukan tabel-tabel yang sudah digunakan dalam permintaan. Ketika anda menambahkan tabel-tabel tambahan melalui parameter table, Django beranggapan anda ingin bahwa tabel disertakan sebuah waktu tambahan, jika itu sudah disertakan. itu membuat sebuah masalah, sejak nama tabel akan kemudian diberikan sebuah nama lain. Jika sebuah tabel muncul berulang kali dalam sebuah pernyataan SQL, kejadian kedua dan seterusnya harus menggunakan nama lain sehingga basisdata dapat membedakan mereka. Jika anda sedang mengacu pada tabel tambahan anda telah tambahkan dalam parameter where tambahan ini akan menyebabkan kesalahan-kesalahan.

  Secara biasa anda hanya butuh menambahkan tabel tambahan yang belum muncul dalam permintaan. Bagaimanapun, jika kasus diuraikan diatas tidak muncul, ada sedikit pemecahan. Pertama, lihat jika anda dapat mendapatkan dengan tanpa menyertakan tabel tambahan dan menggunakan satu yang sudah ada di dalam permintaan. Jika itu tidak memungkinkan, taruh panggilan extra() anda pada depan dari pembangunan queryset sehingga tabel anda adalah penggunaan pertama dari tabel itu. Akhirnya, jika semuanya gagal, cari permintaan dihasilkan dan tulis kembali tambahan where anda untuk menggunakan nama lain diberikan pada tabel tambahan anda. nama lain akan sama setiap kali anda membangun queryset dalam cara sama, jadi anda dapat bergantung pada nama lain untuk tidak berubah.

• order_by

  If you need to order the resulting queryset using some of the new fields or tables you have included via extra() use the order_by parameter to extra() and pass in a sequence of strings. These strings should either be model fields (as in the normal order_by() method on querysets), of the form table_name.column_name or an alias for a column that you specified in the select parameter to extra().

  Sebagai contoh:

  q = Entry.objects.extra(select={'is_recent': "pub_date > '2006-01-01'"})
  q = q.extra(order_by = ['-is_recent'])
  

  This would sort all the items for which is_recent is true to the front of the result set (True sorts before False in a descending ordering).

  This shows, by the way, that you can make multiple calls to extra() and it will behave as you expect (adding new constraints each time).

• params

  The where parameter described above may use standard Python database string placeholders — '%s' to indicate parameters the database engine should automatically quote. The params argument is a list of any extra parameters to be substituted.

  Contoh:

  Entry.objects.extra(where=['headline=%s'], params=['Lennon'])
  

  Always use params instead of embedding values directly into where because params will ensure values are quoted correctly according to your particular backend. For example, quotes will be escaped correctly.

  Buruk:

  Entry.objects.extra(where=["headline='Lennon'"])
  

  Baik:

  Entry.objects.extra(where=['headline=%s'], params=['Lennon'])
  

Peringatan

Jika anda sedang melakukan permintaan pada MySQL, catat bahwa paksaan jenis diam MySQL mungkin menyebabkan hasil tidak diharapkan ketika mencampur jenis. Jika anda meminta pada sebuah kolom jenis string, tetapi dengan sebuah nilai integer, MySQL akan memaksa jenis-jenis dari semua nilai dalam tabel ke integer sebelum melakukan perbandingan. Sebagai contoh, jika tabel anda mengandung nilai-nilai 'abc', 'def' dan anda meminta untuk WHERE mycolumn=0, kedua baris akan cocok. Untuk mencegah ini, lakukan pembenaran typecast sebelum menggunakan nilai dalam sebuah permintaan.

defer()

defer(*fields)

Dalam beberapa keadaan permodelan-data rumit, model anda mungkin mengandung banyak bidang, beberapa diantaranya dapat mengandung banyak data (sebagai contoh, bidang teks), atau mewajibkan pengolahan mahal untuk merubah mereka jadi obyek Python. Jika anda sedang menggunakan hasil dari queryset dalam beberapa keadaan dimana anda tidak mengetahui jika bidang-bidang tertentu tersebut ketika anda menginisialisasikan mengambil data, anda dapat memberitahu Django tidak mengambil mereka dari basisdata.

This is done by passing the names of the fields to not load to defer():

Entry.objects.defer("headline", "body")

A queryset that has deferred fields will still return model instances. Each deferred field will be retrieved from the database if you access that field (one at a time, not all the deferred fields at once).

Catatan

Deferred fields will not lazy-load like this from asynchronous code. Instead, you will get a SynchronousOnlyOperation exception. If you are writing asynchronous code, you should not try to access any fields that you defer().

Anda dapat membuat panggilan banyak pada defer(). Setiap panggilan menambahkan bidang baru ke kumpulan yang ditangguhkan:

# Defers both the body and headline fields.
Entry.objects.defer("body").filter(rating=5).defer("headline")

The order in which fields are added to the deferred set does not matter. Calling defer() with a field name that has already been deferred is harmless (the field will still be deferred).

You can defer loading of fields in related models (if the related models are loading via select_related()) by using the standard double-underscore notation to separate related fields:

Blog.objects.select_related().defer("entry__headline", "entry__body")

Jika anda ingin membersihkan kumpulan dari bidang-bidang yang ditunda, lewatkan None sebagai sebuah parameter pada deferr().

# Load all fields immediately.
my_queryset.defer(None)

Beberapa bidang dalam sebuah model tidak aakan ditunda, bahkan jika anda meminta mereka. Anda tidak pernah dapat menunda memuat primary key. Jika anda sedang menggunakan select_related() mengambil model terkait, anda tidak harus memunda dari bidang yang terhubung ke model utama ke satu terkait, melakukannya akan menghasilkan dalam sebuah kesalahan.

Catatan

Metode defer() (dan sepupunya, only(), dibawah) hanya untuk kasus-penggunaan lebih lanjut. Mereka menyediakan sebuah optimalisasi untuk kapan anda telah menganalisa permintaan anda lebih dekat dan memahami tepatnya informasi apa anda butuhkan dan telah diukur bahwa perbedaan diantara mengembalikan bidang-bidang anda butuh dan kumpulan penuh dari model akan lebih penting.

Even if you think you are in the advanced use-case situation, only use defer() when you cannot, at queryset load time, determine if you will need the extra fields or not. If you are frequently loading and using a particular subset of your data, the best choice you can make is to normalize your models and put the non-loaded data into a separate model (and database table). If the columns must stay in the one table for some reason, create a model with Meta.managed = False (see the managed attribute documentation) containing just the fields you normally need to load and use that where you might otherwise call defer(). This makes your code more explicit to the reader, is slightly faster and consumes a little less memory in the Python process.

Sebagai contoh, kedua model ini menggunakan tabel basisdata pokok sama:

class CommonlyUsedModel(models.Model):
    f1 = models.CharField(max_length=10)

    class Meta:
        managed = False
        db_table = 'app_largetable'

class ManagedModel(models.Model):
    f1 = models.CharField(max_length=10)
    f2 = models.CharField(max_length=10)

    class Meta:
        db_table = 'app_largetable'

# Two equivalent QuerySets:
CommonlyUsedModel.objects.all()
ManagedModel.objects.defer('f2')

If many fields need to be duplicated in the unmanaged model, it may be best to create an abstract model with the shared fields and then have the unmanaged and managed models inherit from the abstract model.

Catatan

Ketika memanggil save() sebagai contoh dengan bidang-bidang tertangguh, hanya bidang-bidang termuat akan disimpan. Lihat save() untuk rincian lebih.

only()

only(*fields)

The only() method is essentially the opposite of defer(). Only the fields passed into this method and that are not already specified as deferred are loaded immediately when the queryset is evaluated.

If you have a model where almost all the fields need to be deferred, using only() to specify the complementary set of fields can result in simpler code.

Suppose you have a model with fields name, age and biography. The following two querysets are the same, in terms of deferred fields:

Person.objects.defer("age", "biography")
Person.objects.only("name")

Whenever you call only() it replaces the set of fields to load immediately. The method's name is mnemonic: only those fields are loaded immediately; the remainder are deferred. Thus, successive calls to only() result in only the final fields being considered:

# This will defer all fields except the headline.
Entry.objects.only("body", "rating").only("headline")

Since defer() acts incrementally (adding fields to the deferred list), you can combine calls to only() and defer() and things will behave logically:

# Final result is that everything except "headline" is deferred.
Entry.objects.only("headline", "body").defer("body")

# Final result loads headline immediately.
Entry.objects.defer("body").only("headline", "body")

All of the cautions in the note for the defer() documentation apply to only() as well. Use it cautiously and only after exhausting your other options.

Menggunakan only() dan memancarkan sebuah bidang diminta menggunakan select_related() adalah sebuah kesalahan juga.

As with defer(), you cannot access the non-loaded fields from asynchronous code and expect them to load. Instead, you will get a SynchronousOnlyOperation exception. Ensure that all fields you might access are in your only() call.

Catatan

Ketika memanggil save() sebagai contoh dengan bidang-bidang tertangguh, hanya bidang-bidang termuat akan disimpan. Lihat save() untuk rincian lebih.

Catatan

When using defer() after only() the fields in defer() will override only() for fields that are listed in both.

using()

using(alias)

This method is for controlling which database the QuerySet will be evaluated against if you are using more than one database. The only argument this method takes is the alias of a database, as defined in DATABASES.

Sebagai contoh:

# queries the database with the 'default' alias.
>>> Entry.objects.all()

# queries the database with the 'backup' alias
>>> Entry.objects.using('backup')

select_for_update()

select_for_update(nowait=False, skip_locked=False, of=(), no_key=False)

Returns a queryset that will lock rows until the end of the transaction, generating a SELECT ... FOR UPDATE SQL statement on supported databases.

Sebagai contoh:

from django.db import transaction

entries = Entry.objects.select_for_update().filter(author=request.user)
with transaction.atomic():
    for entry in entries:
        ...

When the queryset is evaluated (for entry in entries in this case), all matched entries will be locked until the end of the transaction block, meaning that other transactions will be prevented from changing or acquiring locks on them.

Usually, if another transaction has already acquired a lock on one of the selected rows, the query will block until the lock is released. If this is not the behavior you want, call select_for_update(nowait=True). This will make the call non-blocking. If a conflicting lock is already acquired by another transaction, DatabaseError will be raised when the queryset is evaluated. You can also ignore locked rows by using select_for_update(skip_locked=True) instead. The nowait and skip_locked are mutually exclusive and attempts to call select_for_update() with both options enabled will result in a ValueError.

By default, select_for_update() locks all rows that are selected by the query. For example, rows of related objects specified in select_related() are locked in addition to rows of the queryset's model. If this isn't desired, specify the related objects you want to lock in select_for_update(of=(...)) using the same fields syntax as select_related(). Use the value 'self' to refer to the queryset's model.

Lock parents models in select_for_update(of=(...))

If you want to lock parents models when using multi-table inheritance, you must specify parent link fields (by default <parent_model_name>_ptr) in the of argument. For example:

Restaurant.objects.select_for_update(of=('self', 'place_ptr'))

Using select_for_update(of=(...)) with specified fields

If you want to lock models and specify selected fields, e.g. using values(), you must select at least one field from each model in the of argument. Models without selected fields will not be locked.

On PostgreSQL only, you can pass no_key=True in order to acquire a weaker lock, that still allows creating rows that merely reference locked rows (through a foreign key, for example) while the lock is in place. The PostgreSQL documentation has more details about row-level lock modes.

Anda tidak dapat menggunakan select_for_update() pada hubungan null:

>>> Person.objects.select_related('hometown').select_for_update()
Traceback (most recent call last):
...
django.db.utils.NotSupportedError: FOR UPDATE cannot be applied to the nullable side of an outer join

Untuk menghindari pembatasan, anda dapat tidak menyertakan obyek null jika anda tidak peduli dengan mereka:

>>> Person.objects.select_related('hometown').select_for_update().exclude(hometown=None)
<QuerySet [<Person: ...)>, ...]>

The postgresql, oracle, and mysql database backends support select_for_update(). However, MariaDB only supports the nowait argument, MariaDB 10.6+ also supports the skip_locked argument, and MySQL 8.0.1+ supports the nowait, skip_locked, and of arguments. The no_key argument is only supported on PostgreSQL.

Passing nowait=True, skip_locked=True, no_key=True, or of to select_for_update() using database backends that do not support these options, such as MySQL, raises a NotSupportedError. This prevents code from unexpectedly blocking.

Menilai sebuah queryset dengan select_for_update() dalam suasana autocommit pada backend yang mendukung SELECT ... FOR UPDATE adalah sebuah kesalahan TransactionManagementError karena baris-baris tidak dikunci dalam kasus itu. jiak diizinkan, ini akan memfasilitasi kerusakan data dan dapat dengan mudah disebabkan oleh memanggil kode yang mengharapkan dijalankan dalam sebuah transaksi diluar dari satu.

Menggunakan select_for_update() pada backend yang tidak mendukung SELECT ... FOR UPDATE (seperti SQLite) tidak akan mempunyai pengaruh. SELECT ... FOR UPDATE tidak akan ditambahkan pada permintaan, dan sebuah kesalahan tidak dimunculkan jika select_for_update() digunakan dalam suasana autocommit.

Peringatan

Meskipun select_for_update() biasanya gagal dalam suasana autocommit, sejak TestCase otomatis membungkus setiap percobaan dalam sebuah transaksi, memanggil select_for_update() dalam sebuah TestCase bahkan diluar blok atomic() akan (mungkin tidak diharapkan) lewat tanpa memunculkan sebuah TransactionManagementError. Untuk percobaan dengan benar select_for_update() anda harus menggunakan TransactionTestCase.

Pernyataan tertentu mungkin tidak didukung

PostgreSQL tidak mendukung select_for_update() dengan pernyataan Window.

Changed in Django 4.0:

The skip_locked argument was allowed on MariaDB 10.6+.

raw()

raw(raw_query, params=(), translations=None, using=None)

Takes a raw SQL query, executes it, and returns a django.db.models.query.RawQuerySet instance. This RawQuerySet instance can be iterated over just like a normal QuerySet to provide object instances.

Lihat Melakukan permintaan SQL mentah untuk informasi lebih.

Peringatan

raw() always triggers a new query and doesn't account for previous filtering. As such, it should generally be called from the Manager or from a fresh QuerySet instance.

Penghubung yang mengembalikan``QuerySet`` baru

Queryset paduan harus menggunakan model sama.

AND (&)

Memadukan dua QuerySet menggunakan penghubung AND SQL.

Berikut adalah setara:

Model.objects.filter(x=1) & Model.objects.filter(y=2)
Model.objects.filter(x=1, y=2)
from django.db.models import Q
Model.objects.filter(Q(x=1) & Q(y=2))

Setara SQL:

SELECT ... WHERE x=1 AND y=2

OR (|)

Memadukan dua QuerySet menggunakan penghubung OR SQL.

Berikut adalah setara:

Model.objects.filter(x=1) | Model.objects.filter(y=2)
from django.db.models import Q
Model.objects.filter(Q(x=1) | Q(y=2))

Setara SQL:

SELECT ... WHERE x=1 OR y=2

| is not a commutative operation, as different (though equivalent) queries may be generated.

XOR (^)

New in Django 4.1.

Combines two QuerySets using the SQL XOR operator.

Berikut adalah setara:

Model.objects.filter(x=1) ^ Model.objects.filter(y=2)
from django.db.models import Q
Model.objects.filter(Q(x=1) ^ Q(y=2))

Setara SQL:

SELECT ... WHERE x=1 XOR y=2

Catatan

XOR is natively supported on MariaDB and MySQL. On other databases, x ^ y ^ ... ^ z is converted to an equivalent:

(x OR y OR ... OR z) AND
1=(
    (CASE WHEN x THEN 1 ELSE 0 END) +
    (CASE WHEN y THEN 1 ELSE 0 END) +
    ...
    (CASE WHEN z THEN 1 ELSE 0 END) +
)

Metode-metode yang tidak mengembalikan QuerySet

Metode QuerySet berikut menilai QuerySet dan mengembalikan sesuatu selain dari QuerySet.

Metode-metode ini tidak menggunakan penyimpanan sementara (lihat Cache dan QuerySet). Daripada, mereka meminta basisdata setiap kali mereka dipanggil.

Because these methods evaluate the QuerySet, they are blocking calls, and so their main (synchronous) versions cannot be called from asynchronous code. For this reason, each has a corresponding asynchronous version with an a prefix - for example, rather than get(…) you can await aget(…).

There is usually no difference in behavior apart from their asynchronous nature, but any differences are noted below next to each method.

Changed in Django 4.1:

The asynchronous versions of each method, prefixed with a was added.

get()

get(*args, **kwargs)

aget(*args, **kwargs)

Asynchronous version: aget()

Returns the object matching the given lookup parameters, which should be in the format described in Field lookups. You should use lookups that are guaranteed unique, such as the primary key or fields in a unique constraint. For example:

Entry.objects.get(id=1)
Entry.objects.get(Q(blog=blog) & Q(entry_number=1))

If you expect a queryset to already return one row, you can use get() without any arguments to return the object for that row:

Entry.objects.filter(pk=1).get()

If get() doesn't find any object, it raises a Model.DoesNotExist exception:

Entry.objects.get(id=-999) # raises Entry.DoesNotExist

If get() finds more than one object, it raises a Model.MultipleObjectsReturned exception:

Entry.objects.get(name='A Duplicated Name') # raises Entry.MultipleObjectsReturned

Both these exception classes are attributes of the model class, and specific to that model. If you want to handle such exceptions from several get() calls for different models, you can use their generic base classes. For example, you can use django.core.exceptions.ObjectDoesNotExist to handle DoesNotExist exceptions from multiple models:

from django.core.exceptions import ObjectDoesNotExist

try:
    blog = Blog.objects.get(id=1)
    entry = Entry.objects.get(blog=blog, entry_number=1)
except ObjectDoesNotExist:
    print("Either the blog or entry doesn't exist.")

Changed in Django 4.1:

aget() method was added.

create()

create(**kwargs)

acreate(*args, **kwargs)

Asynchronous version: acreate()

Sebuah metode nyaman untuk membuat sebuah obyek dan menyimpan itu semua dalam satu langkah. Jadi:

p = Person.objects.create(first_name="Bruce", last_name="Springsteen")

dan:

p = Person(first_name="Bruce", last_name="Springsteen")
p.save(force_insert=True)

adalah setara.

Parameter force_insert didokumentasikan ditempat lain, tetapi semua itu berarti bahwa sebuah obyek baru akan selalu dibuat. Biasanya anda tidak akan perlu khawatir tentang ini. Bagaimanapun, jika model anda mengandung nilai primary key maual yang anda setel dan jika nilai itu sudah ada dalam basisdata, panggilan pada create() akan gagal denga sebuah IntegrityError karena primary key harus unik. Bersiaplah untuk menangani pengecualian jika anda sedang menggunakan primary keys manual.

Changed in Django 4.1:

acreate() method was added.

get_or_create()

get_or_create(defaults=None, **kwargs)

aget_or_create(defaults=None, **kwargs)

Asynchronous version: aget_or_create()

A convenience method for looking up an object with the given kwargs (may be empty if your model has defaults for all fields), creating one if necessary.

Mengembalikan sebuah tuple dari (object, created), dimana object adalah obyek diambil atau dibuat adalah sebuah boolean menentukan apakah sebuah obyek baru telah dibuat.

This is meant to prevent duplicate objects from being created when requests are made in parallel, and as a shortcut to boilerplatish code. For example:

try:
    obj = Person.objects.get(first_name='John', last_name='Lennon')
except Person.DoesNotExist:
    obj = Person(first_name='John', last_name='Lennon', birthday=date(1940, 10, 9))
    obj.save()

Here, with concurrent requests, multiple attempts to save a Person with the same parameters may be made. To avoid this race condition, the above example can be rewritten using get_or_create() like so:

obj, created = Person.objects.get_or_create(
    first_name='John',
    last_name='Lennon',
    defaults={'birthday': date(1940, 10, 9)},
)

Katakunci argumen apapun dilewatkan ke get_or_create() — kecuali sebuah pilihan dipanggil defaults — akan digunakan dalam panggilan get(). Jika sebuah obyek ditemukan, get_or_create() mengembalikan sebuah tuple dari obyek itu dan False.

Peringatan

This method is atomic assuming that the database enforces uniqueness of the keyword arguments (see unique or unique_together). If the fields used in the keyword arguments do not have a uniqueness constraint, concurrent calls to this method may result in multiple rows with the same parameters being inserted.

You can specify more complex conditions for the retrieved object by chaining get_or_create() with filter() and using Q objects. For example, to retrieve Robert or Bob Marley if either exists, and create the latter otherwise:

from django.db.models import Q

obj, created = Person.objects.filter(
    Q(first_name='Bob') | Q(first_name='Robert'),
).get_or_create(last_name='Marley', defaults={'first_name': 'Bob'})

If multiple objects are found, get_or_create() raises MultipleObjectsReturned. If an object is not found, get_or_create() will instantiate and save a new object, returning a tuple of the new object and True. The new object will be created roughly according to this algorithm:

params = {k: v for k, v in kwargs.items() if '__' not in k}
params.update({k: v() if callable(v) else v for k, v in defaults.items()})
obj = self.model(**params)
obj.save()

Dalam Bahasa Inggris, itu berarti dimulai dengan apapun argumen kata kunci non-'defaults' yang tidak mengandung garis bawah ganda (yang akan menunjukkan sebuah pencarian bukan-persis). Kemudian tambah isi dari defaults, menimpa kunci apapun jika diperlukan, dan gunakan hasil sebagai argumen kata kunci pada kelas model. Jika ada callabke apapun dalam defaults, nilai mereka. Seperti ditunjukkan diatas, ini adalah penyerderhanaan dari algoritma yang digunakan, tetapi itu mengandung semua rincian bersangkutan. Penerapan internal mempunyai beberapa lebih pemeriksanaan-kesalahan daripada ini dan menangani beberapa kondisi-tepi tambahan; jik anda tertarik, baca kode.

If you have a field named defaults and want to use it as an exact lookup in get_or_create(), use 'defaults__exact', like so:

Foo.objects.get_or_create(defaults__exact='bar', defaults={'defaults': 'baz'})

The get_or_create() method has similar error behavior to create() when you're using manually specified primary keys. If an object needs to be created and the key already exists in the database, an IntegrityError will be raised.

Akhirnya, sebuah kata pada penggunakan get_or_create() dalam tampilan Django. Harap pastikan untuk menggunakan itu hanya dalam permintaan POST meskipun anda mempunyai alasan bagus untuk tidak. Permintaan GET tidak harus mempunyai pengaruh pada. Daripada, gunakan POSt kapanpun sebuah permintaan pada sebuah halaman mempunyai pengaruh samping pada data anda. Untuk lebih, lihat Safe methods dalam spesifikasi HTTP.

Peringatan

You can use get_or_create() through ManyToManyField attributes and reverse relations. In that case you will restrict the queries inside the context of that relation. That could lead you to some integrity problems if you don't use it consistently.

Menjadi model berikut:

class Chapter(models.Model):
    title = models.CharField(max_length=255, unique=True)

class Book(models.Model):
    title = models.CharField(max_length=256)
    chapters = models.ManyToManyField(Chapter)

Anda dapat menggunakan get_or_create() melalui bidang bab Book, tetapi itu hanya mengambil dalam konteks dari buku itu:

>>> book = Book.objects.create(title="Ulysses")
>>> book.chapters.get_or_create(title="Telemachus")
(<Chapter: Telemachus>, True)
>>> book.chapters.get_or_create(title="Telemachus")
(<Chapter: Telemachus>, False)
>>> Chapter.objects.create(title="Chapter 1")
<Chapter: Chapter 1>
>>> book.chapters.get_or_create(title="Chapter 1")
# Raises IntegrityError

This is happening because it's trying to get or create "Chapter 1" through the book "Ulysses", but it can't do any of them: the relation can't fetch that chapter because it isn't related to that book, but it can't create it either because title field should be unique.

Changed in Django 4.1:

aget_or_create() method was added.

update_or_create()

update_or_create(defaults=None, **kwargs)

aupdate_or_create(defaults=None, **kwargs)

Asynchronous version: aupdate_or_create()

A convenience method for updating an object with the given kwargs, creating a new one if necessary. The defaults is a dictionary of (field, value) pairs used to update the object. The values in defaults can be callables.

Returns a tuple of (object, created), where object is the created or updated object and created is a boolean specifying whether a new object was created.

The update_or_create method tries to fetch an object from database based on the given kwargs. If a match is found, it updates the fields passed in the defaults dictionary.

This is meant as a shortcut to boilerplatish code. For example:

defaults = {'first_name': 'Bob'}
try:
    obj = Person.objects.get(first_name='John', last_name='Lennon')
    for key, value in defaults.items():
        setattr(obj, key, value)
    obj.save()
except Person.DoesNotExist:
    new_values = {'first_name': 'John', 'last_name': 'Lennon'}
    new_values.update(defaults)
    obj = Person(**new_values)
    obj.save()

This pattern gets quite unwieldy as the number of fields in a model goes up. The above example can be rewritten using update_or_create() like so:

obj, created = Person.objects.update_or_create(
    first_name='John', last_name='Lennon',
    defaults={'first_name': 'Bob'},
)

For a detailed description of how names passed in kwargs are resolved, see get_or_create().

As described above in get_or_create(), this method is prone to a race-condition which can result in multiple rows being inserted simultaneously if uniqueness is not enforced at the database level.

Like get_or_create() and create(), if you're using manually specified primary keys and an object needs to be created but the key already exists in the database, an IntegrityError is raised.

Changed in Django 4.1:

aupdate_or_create() method was added.

bulk_create()

bulk_create(objs, batch_size=None, ignore_conflicts=False, update_conflicts=False, update_fields=None, unique_fields=None)

abulk_create(objs, batch_size=None, ignore_conflicts=False, update_conflicts=False, update_fields=None, unique_fields=None)

Asynchronous version: abulk_create()

This method inserts the provided list of objects into the database in an efficient manner (generally only 1 query, no matter how many objects there are), and returns created objects as a list, in the same order as provided:

>>> objs = Entry.objects.bulk_create([
...     Entry(headline='This is a test'),
...     Entry(headline='This is only a test'),
... ])

Ini mempunyai sejumlah peringatan:

• Metode save() model tidak akan dipanggil, dan sinyal pre_save dan post_save tidak akan dikirim.

• Itu tidak bekerja dengan anak model dalam sebuah skenario warisan banyak-tabel.

• If the model's primary key is an AutoField, the primary key attribute can only be retrieved on certain databases (currently PostgreSQL, MariaDB 10.5+, and SQLite 3.35+). On other databases, it will not be set.

• Itu tidak bekerja dengan hubungan many-to-many.

• It casts objs to a list, which fully evaluates objs if it's a generator. The cast allows inspecting all objects so that any objects with a manually set primary key can be inserted first. If you want to insert objects in batches without evaluating the entire generator at once, you can use this technique as long as the objects don't have any manually set primary keys:

  from itertools import islice
  
  batch_size = 100
  objs = (Entry(headline='Test %s' % i) for i in range(1000))
  while True:
      batch = list(islice(objs, batch_size))
      if not batch:
          break
      Entry.objects.bulk_create(batch, batch_size)
  

The batch_size parameter controls how many objects are created in a single query. The default is to create all objects in one batch, except for SQLite where the default is such that at most 999 variables per query are used.

On databases that support it (all but Oracle), setting the ignore_conflicts parameter to True tells the database to ignore failure to insert any rows that fail constraints such as duplicate unique values.

On databases that support it (all except Oracle and SQLite < 3.24), setting the update_conflicts parameter to True, tells the database to update update_fields when a row insertion fails on conflicts. On PostgreSQL and SQLite, in addition to update_fields, a list of unique_fields that may be in conflict must be provided.

Enabling the ignore_conflicts or update_conflicts parameter disable setting the primary key on each model instance (if the database normally support it).

Peringatan

On MySQL and MariaDB, setting the ignore_conflicts parameter to True turns certain types of errors, other than duplicate key, into warnings. Even with Strict Mode. For example: invalid values or non-nullable violations. See the MySQL documentation and MariaDB documentation for more details.

Changed in Django 4.0:

Support for the fetching primary key attributes on SQLite 3.35+ was added.

Changed in Django 4.1:

The update_conflicts, update_fields, and unique_fields parameters were added to support updating fields when a row insertion fails on conflict.

abulk_create() method was added.

bulk_update()

bulk_update(objs, fields, batch_size=None)

abulk_update(objs, fields, batch_size=None)

Asynchronous version: abulk_update()

This method efficiently updates the given fields on the provided model instances, generally with one query, and returns the number of objects updated:

>>> objs = [
...    Entry.objects.create(headline='Entry 1'),
...    Entry.objects.create(headline='Entry 2'),
... ]
>>> objs[0].headline = 'This is entry 1'
>>> objs[1].headline = 'This is entry 2'
>>> Entry.objects.bulk_update(objs, ['headline'])
2

Changed in Django 4.0:

The return value of the number of objects updated was added.

QuerySet.update() is used to save the changes, so this is more efficient than iterating through the list of models and calling save() on each of them, but it has a few caveats:

• Anda tidak dapat memperbaharui primary key model.
• Each model's save() method isn't called, and the pre_save and post_save signals aren't sent.
• If updating a large number of columns in a large number of rows, the SQL generated can be very large. Avoid this by specifying a suitable batch_size.
• Updating fields defined on multi-table inheritance ancestors will incur an extra query per ancestor.
• When an individual batch contains duplicates, only the first instance in that batch will result in an update.
• The number of objects updated returned by the function may be fewer than the number of objects passed in. This can be due to duplicate objects passed in which are updated in the same batch or race conditions such that objects are no longer present in the database.

The batch_size parameter controls how many objects are saved in a single query. The default is to update all objects in one batch, except for SQLite and Oracle which have restrictions on the number of variables used in a query.

Changed in Django 4.1:

abulk_update() method was added.

count()

count()

acount()

Asynchronous version: acount()

Mengembalikan sebuah integer mewakili sejumlah obyek dalam QuerySet pencocokan basisdata.

Contoh:

# Returns the total number of entries in the database.
Entry.objects.count()

# Returns the number of entries whose headline contains 'Lennon'
Entry.objects.filter(headline__contains='Lennon').count()

A count() call performs a SELECT COUNT(*) behind the scenes, so you should always use count() rather than loading all of the record into Python objects and calling len() on the result (unless you need to load the objects into memory anyway, in which case len() will be faster).

Note that if you want the number of items in a QuerySet and are also retrieving model instances from it (for example, by iterating over it), it's probably more efficient to use len(queryset) which won't cause an extra database query like count() would.

If the queryset has already been fully retrieved, count() will use that length rather than perform an extra database query.

Changed in Django 4.1:

acount() method was added.

in_bulk()

in_bulk(id_list=None, *, field_name='pk')

ain_bulk(id_list=None, *, field_name='pk')

Asynchronous version: ain_bulk()

Takes a list of field values (id_list) and the field_name for those values, and returns a dictionary mapping each value to an instance of the object with the given field value. No django.core.exceptions.ObjectDoesNotExist exceptions will ever be raised by in_bulk; that is, any id_list value not matching any instance will simply be ignored. If id_list isn't provided, all objects in the queryset are returned. field_name must be a unique field or a distinct field (if there's only one field specified in distinct()). field_name defaults to the primary key.

Contoh:

>>> Blog.objects.in_bulk([1])
{1: <Blog: Beatles Blog>}
>>> Blog.objects.in_bulk([1, 2])
{1: <Blog: Beatles Blog>, 2: <Blog: Cheddar Talk>}
>>> Blog.objects.in_bulk([])
{}
>>> Blog.objects.in_bulk()
{1: <Blog: Beatles Blog>, 2: <Blog: Cheddar Talk>, 3: <Blog: Django Weblog>}
>>> Blog.objects.in_bulk(['beatles_blog'], field_name='slug')
{'beatles_blog': <Blog: Beatles Blog>}
>>> Blog.objects.distinct('name').in_bulk(field_name='name')
{'Beatles Blog': <Blog: Beatles Blog>, 'Cheddar Talk': <Blog: Cheddar Talk>, 'Django Weblog': <Blog: Django Weblog>}

Jika anda melewatkan in_bulk() sebuah daftar kosong, anda akan mendapatkan kamus kosong.

Changed in Django 4.1:

ain_bulk() method was added.

iterator()

iterator(chunk_size=None)

aiterator(chunk_size=None)

Asynchronous version: aiterator()

Evaluates the QuerySet (by performing the query) and returns an iterator (see PEP 234) over the results, or an asynchronous iterator (see PEP 492) if you call its asynchronous version aiterator.

A QuerySet typically caches its results internally so that repeated evaluations do not result in additional queries. In contrast, iterator() will read results directly, without doing any caching at the QuerySet level (internally, the default iterator calls iterator() and caches the return value). For a QuerySet which returns a large number of objects that you only need to access once, this can result in better performance and a significant reduction in memory.

Catat bahwa menggunakan iterator() pada sebuah QuerySet yang sudah dinilai akan memaksa itu dinilai kembali, mengulangi permintaan.

iterator() is compatible with previous calls to prefetch_related() as long as chunk_size is given. Larger values will necessitate fewer queries to accomplish the prefetching at the cost of greater memory usage.

Catatan

aiterator() is not compatible with previous calls to prefetch_related().

On some databases (e.g. Oracle, SQLite), the maximum number of terms in an SQL IN clause might be limited. Hence values below this limit should be used. (In particular, when prefetching across two or more relations, a chunk_size should be small enough that the anticipated number of results for each prefetched relation still falls below the limit.)

So long as the QuerySet does not prefetch any related objects, providing no value for chunk_size will result in Django using an implicit default of 2000.

Bergantung pada backend basisdata, hasil permintaan akan salah satu dimuat semua sekali atau dialirkan dari basisdata menggunakan kursor sisi-peladen.

Changed in Django 4.1:

Support for prefetching related objects was added to iterator().

aiterator() method was added.

Ditinggalkan sejak versi 4.1: Using iterator() on a queryset that prefetches related objects without providing the chunk_size is deprecated. In Django 5.0, an exception will be raise.

Dengan jarum sisi-peladen

Oracle dan PostgreSQL menggunakan kursor sisi peladen untuk mengalirkan hasil dari basisdata tanpa memuat keseluruhan hasil disetel kedalam memori.

Driver basisdata Oracle selalu menggunakan jarum sisi-peladen.

With server-side cursors, the chunk_size parameter specifies the number of results to cache at the database driver level. Fetching bigger chunks diminishes the number of round trips between the database driver and the database, at the expense of memory.

On PostgreSQL, server-side cursors will only be used when the DISABLE_SERVER_SIDE_CURSORS setting is False. Read Menggabungkan transaksi dan kursor sisi-peladen if you're using a connection pooler configured in transaction pooling mode. When server-side cursors are disabled, the behavior is the same as databases that don't support server-side cursors.

Tanpa jarum sisi-peladen

MySQL doesn't support streaming results, hence the Python database driver loads the entire result set into memory. The result set is then transformed into Python row objects by the database adapter using the fetchmany() method defined in PEP 249.

SQLite can fetch results in batches using fetchmany(), but since SQLite doesn't provide isolation between queries within a connection, be careful when writing to the table being iterated over. See Pengucilan ketika menggunakan QuerySet.iterator() for more information.

The chunk_size parameter controls the size of batches Django retrieves from the database driver. Larger batches decrease the overhead of communicating with the database driver at the expense of a slight increase in memory consumption.

So long as the QuerySet does not prefetch any related objects, providing no value for chunk_size will result in Django using an implicit default of 2000, a value derived from a calculation on the psycopg mailing list:

Assuming rows of 10-20 columns with a mix of textual and numeric data, 2000 is going to fetch less than 100KB of data, which seems a good compromise between the number of rows transferred and the data discarded if the loop is exited early.

latest()

latest(*fields)

alatest(*fields)

Asynchronous version: alatest()

Mengembalikan obyek terakhir dalam tabel berdasarkan pada bidang-bidang diberikan.

Contoh ini mengembalikan Entry terakhir dalam tabel, menurut pada bidang pub-date:

Entry.objects.latest('pub_date')

Anda dapat juga memilih terakhir berdasarkan pada beberapa bidang. Sebagai contoh, memilih Entry dengan expire_date paling awal ketika dua masukan memiliki pub_date sama:

Entry.objects.latest('pub_date', '-expire_date')

Tanda negatif dalam '-expire_date' berarti mengurutkan expire_date dalam urutan menurun. Karena latest() mendapatkan hasil terakhir, Entry dengan expire_date paling awal yang dipilih.

If your model's Meta specifies get_latest_by, you can omit any arguments to earliest() or latest(). The fields specified in get_latest_by will be used by default.

Seperti get(), earliest() dan latest() memunculkan DoesNotExist jika tidak ada obyek dengan parameter yang diberikan.

Catat bahwa adanya earliest() dan latest() semata-mata untuk kenyamanan dan mudahan membaca.

earliest() dan latest() mungkin mengembalikan instance dengan tanggal null.

Since ordering is delegated to the database, results on fields that allow null values may be ordered differently if you use different databases. For example, PostgreSQL and MySQL sort null values as if they are higher than non-null values, while SQLite does the opposite.

Anda mungkin ingin menyaring nilai-nilai null:

Entry.objects.filter(pub_date__isnull=False).latest('pub_date')

Changed in Django 4.1:

alatest() method was added.

earliest()

earliest(*fields)

aearliest(*fields)

Asynchronous version: aearliest()

Bekerja sebalinya seperti latest() kecuali arah dirubah.

Changed in Django 4.1:

aearliest() method was added.

first()

first()

afirst()

Asynchronous version: afirst()

Returns the first object matched by the queryset, or None if there is no matching object. If the QuerySet has no ordering defined, then the queryset is automatically ordered by the primary key. This can affect aggregation results as described in Interaksi dengan order_by().

Contoh:

p = Article.objects.order_by('title', 'pub_date').first()

catat bahwa first() adalah metode nyaman, contoh kode berikut adalah setara pada contoh diatas:

try:
    p = Article.objects.order_by('title', 'pub_date')[0]
except IndexError:
    p = None

Changed in Django 4.1:

afirst() method was added.

last()

last()

alast()

Asynchronous version: alast()

Bekerja seperti first(), tetapi mengembalikan obyek terakhir dalam queryset.

Changed in Django 4.1:

alast() method was added.

aggregate()

aggregate(*args, **kwargs)

aaggregate(*args, **kwargs)

Asynchronous version: aaggregate()

Returns a dictionary of aggregate values (averages, sums, etc.) calculated over the QuerySet. Each argument to aggregate() specifies a value that will be included in the dictionary that is returned.

The aggregation functions that are provided by Django are described in Aggregation Functions below. Since aggregates are also query expressions, you may combine aggregates with other aggregates or values to create complex aggregates.

Aggregates specified using keyword arguments will use the keyword as the name for the annotation. Anonymous arguments will have a name generated for them based upon the name of the aggregate function and the model field that is being aggregated. Complex aggregates cannot use anonymous arguments and must specify a keyword argument as an alias.

Sebagai contoh, ketika anda sedang bekerja dengan masukan blog, anda mungkin ingin mengetahui sejumlah penulis yang memiliki sumbangan masukan blog:

>>> from django.db.models import Count
>>> q = Blog.objects.aggregate(Count('entry'))
{'entry__count': 16}

Dengan menggunakan argumen kata kunci untuk menentukan fungsi pengumpulan, anda dapat mengendalikan nama dari nilai pengumpulan yang dikembalikan:

>>> q = Blog.objects.aggregate(number_of_entries=Count('entry'))
{'number_of_entries': 16}

For an in-depth discussion of aggregation, see the topic guide on Aggregation.

Changed in Django 4.1:

aaggregate() method was added.

exists()

exists()

aexists()

Asynchronous version: aexists()

Returns True if the QuerySet contains any results, and False if not. This tries to perform the query in the simplest and fastest way possible, but it does execute nearly the same query as a normal QuerySet query.

exists() is useful for searches relating to the existence of any objects in a QuerySet, particularly in the context of a large QuerySet.

To find whether a queryset contains any items:

if some_queryset.exists():
    print("There is at least one object in some_queryset")

Yang akan menjadi lebih cepat daripada:

if some_queryset:
    print("There is at least one object in some_queryset")

... tetapi bukan berdasarkan tingkatan besar (karena itu dibtuuhkan himpunan permintaan besar untuk mendapatkan efisiensi).

Additionally, if a some_queryset has not yet been evaluated, but you know that it will be at some point, then using some_queryset.exists() will do more overall work (one query for the existence check plus an extra one to later retrieve the results) than using bool(some_queryset), which retrieves the results and then checks if any were returned.

Changed in Django 4.1:

aexists() method was added.

contains()

contains(obj)

acontains(obj)

Asynchronous version: acontains()

New in Django 4.0.

Returns True if the QuerySet contains obj, and False if not. This tries to perform the query in the simplest and fastest way possible.

contains() is useful for checking an object membership in a QuerySet, particularly in the context of a large QuerySet.

To check whether a queryset contains a specific item:

if some_queryset.contains(obj):
    print('Entry contained in queryset')

This will be faster than the following which requires evaluating and iterating through the entire queryset:

if obj in some_queryset:
    print('Entry contained in queryset')

Like exists(), if some_queryset has not yet been evaluated, but you know that it will be at some point, then using some_queryset.contains(obj) will make an additional database query, generally resulting in slower overall performance.

Changed in Django 4.1:

acontains() method was added.

update()

update(**kwargs)

aupdate(**kwargs)

Asynchronous version: aupdate()

Performs an SQL update query for the specified fields, and returns the number of rows matched (which may not be equal to the number of rows updated if some rows already have the new value).

Sebagai contoh, untuk mematikan komentar untuk semua masukan blog diterbitkan dalam 2010, anda dapat melakukan ini:

>>> Entry.objects.filter(pub_date__year=2010).update(comments_on=False)

(Ini menganggap model Entry mempunyai bidang pub_date dan comments_on.)

Anda dapat memperbaharui banyak bidang -- tidak ada batasan pada sbeerapa banyak. Sebagai contoh, disini kami memperbaharui bidang-bidang comments_on dan headline.

>>> Entry.objects.filter(pub_date__year=2010).update(comments_on=False, headline='This is old')

The update() method is applied instantly, and the only restriction on the QuerySet that is updated is that it can only update columns in the model's main table, not on related models. You can't do this, for example:

>>> Entry.objects.update(blog__name='foo') # Won't work!

Penyaringan berdasarkan pada bidang terkait masuk mungkin, meskipun:

>>> Entry.objects.filter(blog__id=1).update(comments_on=True)

Anda tidak dapat memanggil update() pada QuerySet yang mempunyai potongan diambil atau dapat sebalinya tidak lagi disaring.

Cara update() mengembalikan sejumlah dari baris yang terpengaruh:

>>> Entry.objects.filter(id=64).update(comments_on=True)
1

>>> Entry.objects.filter(slug='nonexistent-slug').update(comments_on=True)
0

>>> Entry.objects.filter(pub_date__year=2010).update(comments_on=False)
132

If you're just updating a record and don't need to do anything with the model object, the most efficient approach is to call update(), rather than loading the model object into memory. For example, instead of doing this:

e = Entry.objects.get(id=10)
e.comments_on = False
e.save()

...lakukan ini:

Entry.objects.filter(id=10).update(comments_on=False)

Using update() also prevents a race condition wherein something might change in your database in the short period of time between loading the object and calling save().

Akhirnya, sadari bahwa update() melakukan sebuah pembaharuan pada tingkat SQL dan, dengan demikian, tidak memanggil metode save() apapun pada model anda, tidak juga itu mengeluarkan sinyal pre_save atau post_save (yang merupakan konsekuensi dari memanggil Model.save()). Jiak anda ingin memperbaharui segerombol rekaman untuk model yang memmpunyai metode save() penyesuaian, ulangi terhadap mereka dan panggil save(), seperti ini:

for e in Entry.objects.filter(pub_date__year=2010):
    e.comments_on = False
    e.save()

Changed in Django 4.1:

aupdate() method was added.

Ordered queryset

Chaining order_by() with update() is supported only on MariaDB and MySQL, and is ignored for different databases. This is useful for updating a unique field in the order that is specified without conflicts. For example:

Entry.objects.order_by('-number').update(number=F('number') + 1)

Catatan

order_by() clause will be ignored if it contains annotations, inherited fields, or lookups spanning relations.

delete()

delete()

adelete()

Asynchronous version: adelete()

Performs an SQL delete query on all rows in the QuerySet and returns the number of objects deleted and a dictionary with the number of deletions per object type.

delete() diberlakukan cepat. Anda tidak dapat memanggil delete() pada sebuah QuerySet yang mempunyai potongan diambil atau dapat sebaliknya tidak lagi disaring.

Sebagai contoh, untuk menghapus semua masukan dalam blog tertentu:

>>> b = Blog.objects.get(pk=1)

# Delete all the entries belonging to this Blog.
>>> Entry.objects.filter(blog=b).delete()
(4, {'blog.Entry': 2, 'blog.Entry_authors': 2})

By default, Django's ForeignKey emulates the SQL constraint ON DELETE CASCADE — in other words, any objects with foreign keys pointing at the objects to be deleted will be deleted along with them. For example:

>>> blogs = Blog.objects.all()

# This will delete all Blogs and all of their Entry objects.
>>> blogs.delete()
(5, {'blog.Blog': 1, 'blog.Entry': 2, 'blog.Entry_authors': 2})

Kebiasaan turunan ini dapat di sesuaikan melalui argumen on_delete ke ForeignKey.

The delete() method does a bulk delete and does not call any delete() methods on your models. It does, however, emit the pre_delete and post_delete signals for all deleted objects (including cascaded deletions).

Django needs to fetch objects into memory to send signals and handle cascades. However, if there are no cascades and no signals, then Django may take a fast-path and delete objects without fetching into memory. For large deletes this can result in significantly reduced memory usage. The amount of executed queries can be reduced, too.

ForeignKeys which are set to on_delete DO_NOTHING do not prevent taking the fast-path in deletion.

Catat bahwa permintaan dibangkitkan dalam penghapusan obyek adalah sebuah subyek rincian penerapan untuk merubah.

Changed in Django 4.1:

adelete() method was added.

as_manager()

classmethod as_manager()

Class method that returns an instance of Manager with a copy of the QuerySet’s methods. See Membuat pengelola dengan cara QuerySet for more details.

Note that unlike the other entries in this section, this does not have an asynchronous variant as it does not execute a query.

explain()

explain(format=None, **options)

aexplain(format=None, **options)

Asynchronous version: aexplain()

Returns a string of the QuerySet’s execution plan, which details how the database would execute the query, including any indexes or joins that would be used. Knowing these details may help you improve the performance of slow queries.

Sebagai contoh, ketika menggunakan PostgreSQL:

>>> print(Blog.objects.filter(title='My Blog').explain())
Seq Scan on blog  (cost=0.00..35.50 rows=10 width=12)
  Filter: (title = 'My Blog'::bpchar)

Keluaran berbeda secara penting diantara dua basisdata.

explain() didukung oleh semua backend basisdata siap-pakai kecuali oracle karena sebuah penerapan tidak ada langsung.

The format parameter changes the output format from the databases's default, which is usually text-based. PostgreSQL supports 'TEXT', 'JSON', 'YAML', and 'XML' formats. MariaDB and MySQL support 'TEXT' (also called 'TRADITIONAL') and 'JSON' formats. MySQL 8.0.16+ also supports an improved 'TREE' format, which is similar to PostgreSQL's 'TEXT' output and is used by default, if supported.

Beberapa basisdata menerima bendera yang dapat mengembalikan informasi lebih tentang permintaan. Lewatkan bendera-bendera ini sebagai argumen kata kunci. Sebagai contoh, ketika menggunakan PostgreSQL:

>>> print(Blog.objects.filter(title='My Blog').explain(verbose=True, analyze=True))
Seq Scan on public.blog  (cost=0.00..35.50 rows=10 width=12) (actual time=0.004..0.004 rows=10 loops=1)
  Output: id, title
  Filter: (blog.title = 'My Blog'::bpchar)
Planning time: 0.064 ms
Execution time: 0.058 ms

On some databases, flags may cause the query to be executed which could have adverse effects on your database. For example, the ANALYZE flag supported by MariaDB, MySQL 8.0.18+, and PostgreSQL could result in changes to data if there are triggers or if a function is called, even for a SELECT query.

Changed in Django 4.1:

aexplain() method was added.

Pencarian Field

Field lookups are how you specify the meat of an SQL WHERE clause. They're specified as keyword arguments to the QuerySet methods filter(), exclude() and get().

Untuk sebuah perkenalan, lihat models and database queries documentation 1.

Pencarian siap-pakai Dajngo didaftarkan dibawah. Itu juga memungkinkan menulis custom lookups untuk bidang-bidang model.

As a convenience when no lookup type is provided (like in Entry.objects.get(id=14)) the lookup type is assumed to be exact.

exact

Pencocokan tepat. Jika nilai disediakan untuk perbandingan adalah None, itu akan diartikan sebagai sebuah SQL NULL (lihat isnull untuk rincian lebih).

Contoh:

Entry.objects.get(id__exact=14)
Entry.objects.get(id__exact=None)

SQL equivalents:

SELECT ... WHERE id = 14;
SELECT ... WHERE id IS NULL;

Perbandingan MySQL

In MySQL, a database table's "collation" setting determines whether exact comparisons are case-sensitive. This is a database setting, not a Django setting. It's possible to configure your MySQL tables to use case-sensitive comparisons, but some trade-offs are involved. For more information about this, see the collation section in the databases documentation.

iexact

Case-insensitive exact match. If the value provided for comparison is None, it will be interpreted as an SQL NULL (see isnull for more details).

Contoh:

Blog.objects.get(name__iexact='beatles blog')
Blog.objects.get(name__iexact=None)

SQL equivalents:

SELECT ... WHERE name ILIKE 'beatles blog';
SELECT ... WHERE name IS NULL;

Catat permintaan pertama akan cocok 'Beatles Blog', 'beatles blog', 'BeAtLes BLoG', dll.

Pengguna SQLite

When using the SQLite backend and non-ASCII strings, bear in mind the database note about string comparisons. SQLite does not do case-insensitive matching for non-ASCII strings.

contains

Case-sensitive containment test.

Contoh:

Entry.objects.get(headline__contains='Lennon')

Setara SQL:

SELECT ... WHERE headline LIKE '%Lennon%';

Note this will match the headline 'Lennon honored today' but not 'lennon honored today'.

Pengguna SQLite

SQLite doesn't support case-sensitive LIKE statements; contains acts like icontains for SQLite. See the database note for more information.

icontains

Case-insensitive containment test.

Contoh:

Entry.objects.get(headline__icontains='Lennon')

Setara SQL:

SELECT ... WHERE headline ILIKE '%Lennon%';

Pengguna SQLite

When using the SQLite backend and non-ASCII strings, bear in mind the database note about string comparisons.

in

In a given iterable; often a list, tuple, or queryset. It's not a common use case, but strings (being iterables) are accepted.

Contoh:

Entry.objects.filter(id__in=[1, 3, 4])
Entry.objects.filter(headline__in='abc')

SQL equivalents:

SELECT ... WHERE id IN (1, 3, 4);
SELECT ... WHERE headline IN ('a', 'b', 'c');

You can also use a queryset to dynamically evaluate the list of values instead of providing a list of literal values:

inner_qs = Blog.objects.filter(name__contains='Cheddar')
entries = Entry.objects.filter(blog__in=inner_qs)

This queryset will be evaluated as subselect statement:

SELECT ... WHERE blog.id IN (SELECT id FROM ... WHERE NAME LIKE '%Cheddar%')

If you pass in a QuerySet resulting from values() or values_list() as the value to an __in lookup, you need to ensure you are only extracting one field in the result. For example, this will work (filtering on the blog names):

inner_qs = Blog.objects.filter(name__contains='Ch').values('name')
entries = Entry.objects.filter(blog__name__in=inner_qs)

Contoh ini akan memunculkan sebuah pengecualian, sejak permintaan paling dalam mencoba mengeluarkan nilai dua bidang, dimana hanya satu yang diharapkan:

# Bad code! Will raise a TypeError.
inner_qs = Blog.objects.filter(name__contains='Ch').values('name', 'id')
entries = Entry.objects.filter(blog__name__in=inner_qs)

Pertimbangan penampilan

Be cautious about using nested queries and understand your database server's performance characteristics (if in doubt, benchmark!). Some database backends, most notably MySQL, don't optimize nested queries very well. It is more efficient, in those cases, to extract a list of values and then pass that into the second query. That is, execute two queries instead of one:

values = Blog.objects.filter(
        name__contains='Cheddar').values_list('pk', flat=True)
entries = Entry.objects.filter(blog__in=list(values))

Note the list() call around the Blog QuerySet to force execution of the first query. Without it, a nested query would be executed, because QuerySet adalah lazy.

gt

Lebih besar dari

Contoh:

Entry.objects.filter(id__gt=4)

Setara SQL:

SELECT ... WHERE id > 4;

gte

Lebih besar dari atau sama dengan.

lt

Kurang dari

lte

Kurang dari atau sama dengan.

startswith

Dimulai-dengan kasus-peka.

Contoh:

Entry.objects.filter(headline__startswith='Lennon')

Setara SQL:

SELECT ... WHERE headline LIKE 'Lennon%';

SQLite tidak mendukung kasus-peka pernyataan LIKE; startswith bertindak seperti istartswith untuk SQLite.

istartswith

Dimulai-dengan kasus-tidak-peka.

Contoh:

Entry.objects.filter(headline__istartswith='Lennon')

Setara SQL:

SELECT ... WHERE headline ILIKE 'Lennon%';

Pengguna SQLite

When using the SQLite backend and non-ASCII strings, bear in mind the database note about string comparisons.

endswith

Diakhiri-dengan kasus-peka.

Contoh:

Entry.objects.filter(headline__endswith='Lennon')

Setara SQL:

SELECT ... WHERE headline LIKE '%Lennon';

Pengguna SQLite

SQLite doesn't support case-sensitive LIKE statements; endswith acts like iendswith for SQLite. Refer to the database note documentation for more.

iendswith

Diakhiri-dengan kasus-tidak-peka.

Contoh:

Entry.objects.filter(headline__iendswith='Lennon')

Setara SQL:

SELECT ... WHERE headline ILIKE '%Lennon'

Pengguna SQLite

When using the SQLite backend and non-ASCII strings, bear in mind the database note about string comparisons.

range

Jangkauan percobaan (termasuk).

Contoh:

import datetime
start_date = datetime.date(2005, 1, 1)
end_date = datetime.date(2005, 3, 31)
Entry.objects.filter(pub_date__range=(start_date, end_date))

Setara SQL:

SELECT ... WHERE pub_date BETWEEN '2005-01-01' and '2005-03-31';

Anda dapat menggunakan range dimanapun anda dapat gunakan BETWEEN dalam SQL -- untuk tanggal, angka bahkan karakter.

Peringatan

Filtering a DateTimeField with dates won't include items on the last day, because the bounds are interpreted as "0am on the given date". If pub_date was a DateTimeField, the above expression would be turned into this SQL:

SELECT ... WHERE pub_date BETWEEN '2005-01-01 00:00:00' and '2005-03-31 00:00:00';

Secara umum, anda tidak dapat mencampur tanggal dan datetime.

date

Untuk bidang datetime, melemparkan nilai sebagai tanggal. Mengizinkan mengikat pencarian bidang tambahan. Mengambil nilai data.

Contoh:

Entry.objects.filter(pub_date__date=datetime.date(2005, 1, 1))
Entry.objects.filter(pub_date__date__gt=datetime.date(2005, 1, 1))

(No equivalent SQL code fragment is included for this lookup because implementation of the relevant query varies among different database engines.)

When USE_TZ is True, fields are converted to the current time zone before filtering. This requires time zone definitions in the database.

year

Untuk bidang tanggal dan datetime, sebuah kecocokan tanggal tepat. Mengizinkan mengikat tambahan bidang pencarian. Mengambil sebuah tahun integer.

Contoh:

Entry.objects.filter(pub_date__year=2005)
Entry.objects.filter(pub_date__year__gte=2005)

Setara SQL:

SELECT ... WHERE pub_date BETWEEN '2005-01-01' AND '2005-12-31';
SELECT ... WHERE pub_date >= '2005-01-01';

(Sintaksis SQL tepat beragam untuk setiap mesin basisdata.)

When USE_TZ is True, datetime fields are converted to the current time zone before filtering. This requires time zone definitions in the database.

iso_year

For date and datetime fields, an exact ISO 8601 week-numbering year match. Allows chaining additional field lookups. Takes an integer year.

Contoh:

Entry.objects.filter(pub_date__iso_year=2005)
Entry.objects.filter(pub_date__iso_year__gte=2005)

(Sintaksis SQL tepat beragam untuk setiap mesin basisdata.)

When USE_TZ is True, datetime fields are converted to the current time zone before filtering. This requires time zone definitions in the database.

month

Untuk bidang tanggal dan datetime, sebuah kecocokan bulan yang tepat. Mengizinkan mengikat bidang pencarian tambahan. Mengambil sebuah integer 1 (Januari) sampai 12 (Desember).

Contoh:

Entry.objects.filter(pub_date__month=12)
Entry.objects.filter(pub_date__month__gte=6)

Setara SQL:

SELECT ... WHERE EXTRACT('month' FROM pub_date) = '12';
SELECT ... WHERE EXTRACT('month' FROM pub_date) >= '6';

(Sintaksis SQL tepat beragam untuk setiap mesin basisdata.)

When USE_TZ is True, datetime fields are converted to the current time zone before filtering. This requires time zone definitions in the database.

day

For date and datetime fields, an exact day match. Allows chaining additional field lookups. Takes an integer day.

Contoh:

Entry.objects.filter(pub_date__day=3)
Entry.objects.filter(pub_date__day__gte=3)

Setara SQL:

SELECT ... WHERE EXTRACT('day' FROM pub_date) = '3';
SELECT ... WHERE EXTRACT('day' FROM pub_date) >= '3';

(Sintaksis SQL tepat beragam untuk setiap mesin basisdata.)

Catat ini akan cocok dengan rekaman apapun dengan pub_date pada ahri ketiga dari bulan, seperti 3 Januari, 3 Juli, dll.

When USE_TZ is True, datetime fields are converted to the current time zone before filtering. This requires time zone definitions in the database.

week

For date and datetime fields, return the week number (1-52 or 53) according to ISO-8601, i.e., weeks start on a Monday and the first week contains the year's first Thursday.

Contoh:

Entry.objects.filter(pub_date__week=52)
Entry.objects.filter(pub_date__week__gte=32, pub_date__week__lte=38)

(No equivalent SQL code fragment is included for this lookup because implementation of the relevant query varies among different database engines.)

When USE_TZ is True, datetime fields are converted to the current time zone before filtering. This requires time zone definitions in the database.

week_day

Untuk bidang tanggal dan datetime, pencocokan 'hari dari minggu'. Mengizinkan mengikat bidang pencarian tambahan.

Mengambil nilai integer mewkili hari dari minggu dari 1 (Minggu) sampai 7 (Sabtu).

Contoh:

Entry.objects.filter(pub_date__week_day=2)
Entry.objects.filter(pub_date__week_day__gte=2)

(No equivalent SQL code fragment is included for this lookup because implementation of the relevant query varies among different database engines.)

Note this will match any record with a pub_date that falls on a Monday (day 2 of the week), regardless of the month or year in which it occurs. Week days are indexed with day 1 being Sunday and day 7 being Saturday.

When USE_TZ is True, datetime fields are converted to the current time zone before filtering. This requires time zone definitions in the database.

iso_week_day

For date and datetime fields, an exact ISO 8601 day of the week match. Allows chaining additional field lookups.

Takes an integer value representing the day of the week from 1 (Monday) to 7 (Sunday).

Contoh:

Entry.objects.filter(pub_date__iso_week_day=1)
Entry.objects.filter(pub_date__iso_week_day__gte=1)

(No equivalent SQL code fragment is included for this lookup because implementation of the relevant query varies among different database engines.)

Note this will match any record with a pub_date that falls on a Monday (day 1 of the week), regardless of the month or year in which it occurs. Week days are indexed with day 1 being Monday and day 7 being Sunday.

When USE_TZ is True, datetime fields are converted to the current time zone before filtering. This requires time zone definitions in the database.

quarter

For date and datetime fields, a 'quarter of the year' match. Allows chaining additional field lookups. Takes an integer value between 1 and 4 representing the quarter of the year.

Contoh mengambil masukan dalam perempat kedua (1 April sampai 30 Juni):

Entry.objects.filter(pub_date__quarter=2)

(No equivalent SQL code fragment is included for this lookup because implementation of the relevant query varies among different database engines.)

When USE_TZ is True, datetime fields are converted to the current time zone before filtering. This requires time zone definitions in the database.

time

Untuk bidang datetime, melemparkan nilai sebagai waktu. Mengizinkan pencarian bidang tambahan. Mengambil nilai datetime.time.

Contoh:

Entry.objects.filter(pub_date__time=datetime.time(14, 30))
Entry.objects.filter(pub_date__time__range=(datetime.time(8), datetime.time(17)))

(No equivalent SQL code fragment is included for this lookup because implementation of the relevant query varies among different database engines.)

When USE_TZ is True, fields are converted to the current time zone before filtering. This requires time zone definitions in the database.

hour

For datetime and time fields, an exact hour match. Allows chaining additional field lookups. Takes an integer between 0 and 23.

Contoh:

Event.objects.filter(timestamp__hour=23)
Event.objects.filter(time__hour=5)
Event.objects.filter(timestamp__hour__gte=12)

Setara SQL:

SELECT ... WHERE EXTRACT('hour' FROM timestamp) = '23';
SELECT ... WHERE EXTRACT('hour' FROM time) = '5';
SELECT ... WHERE EXTRACT('hour' FROM timestamp) >= '12';

(Sintaksis SQL tepat beragam untuk setiap mesin basisdata.)

When USE_TZ is True, datetime fields are converted to the current time zone before filtering. This requires time zone definitions in the database.

minute

For datetime and time fields, an exact minute match. Allows chaining additional field lookups. Takes an integer between 0 and 59.

Contoh:

Event.objects.filter(timestamp__minute=29)
Event.objects.filter(time__minute=46)
Event.objects.filter(timestamp__minute__gte=29)

Setara SQL:

SELECT ... WHERE EXTRACT('minute' FROM timestamp) = '29';
SELECT ... WHERE EXTRACT('minute' FROM time) = '46';
SELECT ... WHERE EXTRACT('minute' FROM timestamp) >= '29';

(Sintaksis SQL tepat beragam untuk setiap mesin basisdata.)

When USE_TZ is True, datetime fields are converted to the current time zone before filtering. This requires time zone definitions in the database.

second

For datetime and time fields, an exact second match. Allows chaining additional field lookups. Takes an integer between 0 and 59.

Contoh:

Event.objects.filter(timestamp__second=31)
Event.objects.filter(time__second=2)
Event.objects.filter(timestamp__second__gte=31)

Setara SQL:

SELECT ... WHERE EXTRACT('second' FROM timestamp) = '31';
SELECT ... WHERE EXTRACT('second' FROM time) = '2';
SELECT ... WHERE EXTRACT('second' FROM timestamp) >= '31';

(Sintaksis SQL tepat beragam untuk setiap mesin basisdata.)

When USE_TZ is True, datetime fields are converted to the current time zone before filtering. This requires time zone definitions in the database.

isnull

Takes either True or False, which correspond to SQL queries of IS NULL and IS NOT NULL, respectively.

Contoh:

Entry.objects.filter(pub_date__isnull=True)

Setara SQL:

SELECT ... WHERE pub_date IS NULL;

regex

Pencocokan regular expression kasus-peka.

The regular expression syntax is that of the database backend in use. In the case of SQLite, which has no built in regular expression support, this feature is provided by a (Python) user-defined REGEXP function, and the regular expression syntax is therefore that of Python's re module.

Contoh:

Entry.objects.get(title__regex=r'^(An?|The) +')

SQL equivalents:

SELECT ... WHERE title REGEXP BINARY '^(An?|The) +'; -- MySQL

SELECT ... WHERE REGEXP_LIKE(title, '^(An?|The) +', 'c'); -- Oracle

SELECT ... WHERE title ~ '^(An?|The) +'; -- PostgreSQL

SELECT ... WHERE title REGEXP '^(An?|The) +'; -- SQLite

Menggunakan string mentah (misalnya, r'foo' daripada 'foo') untuk melewatkan sintaksis regular expression adalah dianjurkan.

iregex

Pencocokan regular expression kasus-tidak-peka.

Contoh:

Entry.objects.get(title__iregex=r'^(an?|the) +')

SQL equivalents:

SELECT ... WHERE title REGEXP '^(an?|the) +'; -- MySQL

SELECT ... WHERE REGEXP_LIKE(title, '^(an?|the) +', 'i'); -- Oracle

SELECT ... WHERE title ~* '^(an?|the) +'; -- PostgreSQL

SELECT ... WHERE title REGEXP '(?i)^(an?|the) +'; -- SQLite

Fungsi-fungsi pengumpulan

Django provides the following aggregation functions in the django.db.models module. For details on how to use these aggregate functions, see the topic guide on aggregation. See the Aggregate documentation to learn how to create your aggregates.

Peringatan

SQLite can't handle aggregation on date/time fields out of the box. This is because there are no native date/time fields in SQLite and Django currently emulates these features using a text field. Attempts to use aggregation on date/time fields in SQLite will raise NotSupportedError.

Catatan

Aggregation functions return None when used with an empty QuerySet. For example, the Sum aggregation function returns None instead of 0 if the QuerySet contains no entries. To return another value instead, pass a value to the default argument. An exception is Count, which does return 0 if the QuerySet is empty. Count does not support the default argument.

Semua pengumpulan mempunyai parameter berikut secara umum:

expressions

Strings that reference fields on the model, transforms of the field, or query expressions.

output_field

Sebuah argumen pilihan yang mewakili model field 1 dari nilai balikan

Catatan

When combining multiple field types, Django can only determine the output_field if all fields are of the same type. Otherwise, you must provide the output_field yourself.

filter

Sebuah pilihan Q object yang digunakan untuk menyaring baris yang dikumpulkan.

Lihat Pengumpulan bersyarat and Penyaringan pada keterangan untuk contoh penggunaan.

default

New in Django 4.0.

An optional argument that allows specifying a value to use as a default value when the queryset (or grouping) contains no entries.

**extra

Argumen kata kunci yang dapat menyediakan isi tambahan untuk SQL dibangkitkan dengan pengumpulan.

Avg

class Avg(expression, output_field=None, distinct=False, filter=None, default=None, **extra)

Mengembalikan arti nilai dari pernyataan yang diberikan, yang harus berupa numerik meskipun anda menentukan sebuah output_field berbeda.

• Nama lain awalan: 1__avg
• Return type: float if input is int, otherwise same as input field, or output_field if supplied

distinct

Optional. If distinct=True, Avg returns the mean value of unique values. This is the SQL equivalent of AVG(DISTINCT <field>). The default value is False.

Count

class Count(expression, distinct=False, filter=None, **extra)

Mengembalikan sejumlah obyek yang terkait melalui pernyataan yang disediakan.

• Nama lain awalan: 1__count
• Jenis kembalian: int

distinct

Optional. If distinct=True, the count will only include unique instances. This is the SQL equivalent of COUNT(DISTINCT <field>). The default value is False.

Catatan

The default argument is not supported.

Max

class Max(expression, output_field=None, filter=None, default=None, **extra)

Mengembalikan nilai maksimal dari pernyataan yang diberikan.

• Nama lain awalan: 1__max
• Jenis balikan: sama seperti bidang masukan, atau output_field jika didukung

Min

class Min(expression, output_field=None, filter=None, default=None, **extra)

Mengembalikan nilai minimal dari pernyataan yang diberikan.

• Nama lain awalan: 1__min
• Jenis balikan: sama seperti bidang masukan, atau output_field jika didukung

StdDev

class StdDev(expression, output_field=None, sample=False, filter=None, default=None, **extra)

Mengembalikan selisih standar dari data dalam pernyataan yang disediakan.

• Nama lain awalan: 1__stddev
• Return type: float if input is int, otherwise same as input field, or output_field if supplied

sample

Optional. By default, StdDev returns the population standard deviation. However, if sample=True, the return value will be the sample standard deviation.

Sum

class Sum(expression, output_field=None, distinct=False, filter=None, default=None, **extra)

Menghitung jumlah dari semua nilai dari pernyataan yang diberikan.

• Nama lain awalan: 1__sum
• Jenis balikan: sama seperti bidang masukan, atau output_field jika didukung

distinct

Optional. If distinct=True, Sum returns the sum of unique values. This is the SQL equivalent of SUM(DISTINCT <field>). The default value is False.

Variance

class Variance(expression, output_field=None, sample=False, filter=None, default=None, **extra)

Mengembalikan beragam dari data di pernyataan yang disediakan.

• Nama lain awalan: 1__variance
• Return type: float if input is int, otherwise same as input field, or output_field if supplied

sample

Optional. By default, Variance returns the population variance. However, if sample=True, the return value will be the sample variance.

Alat-alat terkait-permintaan

Bagian ini menyediakan bahanacuan untuk alat-alat terkait-permintaan tidak didokumentasikan dimanapun.

Obyek Q()

class Q

A Q() object represents an SQL condition that can be used in database-related operations. It's similar to how an F() object represents the value of a model field or annotation. They make it possible to define and reuse conditions, and combine them using operators such as | (OR), & (AND), and ^ (XOR). See Pencarian rumit dengan obyek Q.

Changed in Django 4.1:

Support for the ^ (XOR) operator was added.

Obyek Prefetch()

class Prefetch(lookup, queryset=None, to_attr=None)

Obyek Prefetch() dapat digunakan mengendalikan tindakan dari prefetch_related().

Argumen lookup menggambarkan hubungan untuk mengikuti dan bekerja sama seperti pencarian berdasarkan string dilewatkan ke prefetch_related(). Sebagai contoh:

>>> from django.db.models import Prefetch
>>> Question.objects.prefetch_related(Prefetch('choice_set')).get().choice_set.all()
<QuerySet [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]>
# This will only execute two queries regardless of the number of Question
# and Choice objects.
>>> Question.objects.prefetch_related(Prefetch('choice_set'))
<QuerySet [<Question: What's up?>]>

Argumen queryset memasok dasar QuerySet untuk pencarian diberikan. Ini berguna untuk menyaring lebih lanjut tindakan pengambilan, atau memanggil select_related() dari hubungan pengambilan, dengan demikian mengurangi sejumlah permintaan lebih jauh:

>>> voted_choices = Choice.objects.filter(votes__gt=0)
>>> voted_choices
<QuerySet [<Choice: The sky>]>
>>> prefetch = Prefetch('choice_set', queryset=voted_choices)
>>> Question.objects.prefetch_related(prefetch).get().choice_set.all()
<QuerySet [<Choice: The sky>]>

Argumen to_attr menyetel hasil daro tindakan pengambilan pada atribut penyesuaian:

>>> prefetch = Prefetch('choice_set', queryset=voted_choices, to_attr='voted_choices')
>>> Question.objects.prefetch_related(prefetch).get().voted_choices
[<Choice: The sky>]
>>> Question.objects.prefetch_related(prefetch).get().choice_set.all()
<QuerySet [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]>

Catatan

Ketika menggunakan to_attr hasil pengambilan disimpan dalam sebuah daftar. Ini dapat menyediakan peningkatan kecepatan yang signifikan terhadap panggilan prefetch_related tradisional yang menyimpan hasil cache dalam sebuah instance QuerySet.

prefetch_related_objects()

prefetch_related_objects(model_instances, *related_lookups)

Pengambilan pencarian diberikan pada perulangan instance model. Ini berguna dalam kode yang menerima sebuah daftar dari instance model sebagai lawan pada sebuah QuerySet; sebagai contoh, ketika mengambil model-model dari sebuah cache atau menginstansiasi mereka manual.

Lewatkan sebuah perulangan dari instance model (harus semua jadi kelas sama) dan pencarian atau obyek Prefetch anda ingin ambil. Sebagai contoh:

>>> from django.db.models import prefetch_related_objects
>>> restaurants = fetch_top_restaurants_from_cache()  # A list of Restaurants
>>> prefetch_related_objects(restaurants, 'pizzas__toppings')

When using multiple databases with prefetch_related_objects, the prefetch query will use the database associated with the model instance. This can be overridden by using a custom queryset in a related lookup.

Obyek FilteredRelation()

class FilteredRelation(relation_name, *, condition=Q())

relation_name

Nama dari bidang dimana anda ingin saring hubungan.

condition

Sebuah obyek Q untuk mengendalikan penyaringan.

FilteredRelation is used with annotate() to create an ON clause when a JOIN is performed. It doesn't act on the default relationship but on the annotation name (pizzas_vegetarian in example below).

Sebagai contoh, untuk menemukan restoran yang memiliki pizza vegetarian dengan nama 'mozzarella':

>>> from django.db.models import FilteredRelation, Q
>>> Restaurant.objects.annotate(
...    pizzas_vegetarian=FilteredRelation(
...        'pizzas', condition=Q(pizzas__vegetarian=True),
...    ),
... ).filter(pizzas_vegetarian__name__icontains='mozzarella')

Jika ada sejumlah besar pizza, queryset ini melakukan lebih baik dari:

>>> Restaurant.objects.filter(
...     pizzas__vegetarian=True,
...     pizzas__name__icontains='mozzarella',
... )

karena penyaringan dalam klausa WHERE dari himpunan permintaan pertama hanya akan mengoperasikan pada pizza vegetarian.

FilteredRelation tidak mendukung:

• QuerySet.only() dan prefetch_related().
• Sebuah GenericForeignKey diwarisi dari model induk.