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 usingasync for
:async for e in Entry.objects.all(): results.append(e)
Both synchronous and asynchronous iterators of QuerySets share the same underlying cache.
Slicing. As explained in Membatasi QuerySet, a
QuerySet
can be sliced, using Python's array-slicing syntax. Slicing an unevaluatedQuerySet
usually returns another unevaluatedQuerySet
, but Django will execute the database query if you use the "step" parameter of slice syntax, and will return a list. Slicing aQuerySet
that has been evaluated also returns a list.Also note that even though slicing an unevaluated
QuerySet
returns another unevaluatedQuerySet
, 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 callrepr()
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 calllen()
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 acount()
method for precisely this reason.list(). Force evaluation of a
QuerySet
by callinglist()
on it. For example:entry_list = list(Entry.objects.all())
bool(). Testing a
QuerySet
in a boolean context, such as usingbool()
,or
,and
or anif
statement, will cause the query to be executed. If there is at least one result, theQuerySet
isTrue
, otherwiseFalse
. 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 QuerySet
s¶
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'}]>
API QuerySet
¶
Here's the formal declaration of a QuerySet
:
- class QuerySet(model=None, query=None, using=None, hints=None)[sumber]¶
Usually when you'll interact with a
QuerySet
you'll use it by chaining filters. To make this work, mostQuerySet
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[sumber]¶
True
if theQuerySet
is ordered — i.e. has anorder_by()
clause or a default ordering on the model.False
otherwise.
Catatan
The
query
parameter toQuerySet
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.
Examples (those after the first will only work on 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.
This example compares the dictionaries of values()
with the normal model
objects:
# 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.
Example:
>>> 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 aForeignKey
, the defaultvalues()
call will return a dictionary key calledfoo_id
, since this is the name of the hidden model attribute that stores the actual value (thefoo
attribute refers to the related model). When you are callingvalues()
and passing in field names, you can pass in eitherfoo
orfoo_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 withdistinct()
, be aware that ordering can affect the results. See the note indistinct()
for details.If you use a
values()
clause after anextra()
call, any fields defined by aselect
argument in theextra()
must be explicitly included in thevalues()
call. Anyextra()
call made after avalues()
call will have its extra selected fields ignored.Calling
only()
anddefer()
aftervalues()
doesn't make sense, so doing so will raise aTypeError
.Combining transforms and aggregates requires the use of two
annotate()
calls, either explicitly or as keyword arguments tovalues()
. As above, if the transform has been registered on the relevant field type the firstannotate()
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 1-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)¶
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
.
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
QuerySet
s. 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 QuerySet
s
of other models. Passing different models works as long as the SELECT
list
is the same in all QuerySet
s (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
QuerySet
s. 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 QuerySet
s. For example:
>>> qs1.difference(qs2, qs3)
Lihat union()
untuk beberapa pembatasan.
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.
For example, this use of extra()
:
>>> qs.extra(
... select={"val": "select col from sometable where othercol = %s"},
... select_params=(someparam,),
... )
is equivalent to:
>>> 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 klausaSELECT
. 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 apakahpub_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 atributentry_count
, sebuah hitungan integer dari obyekEntry
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 itsFROM
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.In some rare cases, you might wish to pass parameters to the SQL fragments in
extra(select=...)
. For this purpose, use theselect_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 usingwhere
. You can manually add tables to the SQLFROM
clause by usingtables
.where
andtables
both take a list of strings. Allwhere
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 parametertable
, 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 parameterwhere
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 tambahanwhere
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 theorder_by
parameter toextra()
and pass in a sequence of strings. These strings should either be model fields (as in the normalorder_by()
method on querysets), of the formtable_name.column_name
or an alias for a column that you specified in theselect
parameter toextra()
.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 beforeFalse
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. Theparams
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 intowhere
becauseparams
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.
Similarly, calling defer()
(or its counterpart only()
) including an
argument from an aggregation (e.g. using the result of annotate()
)
doesn't make sense: doing so will raise an exception. The aggregated values
will always be fetched into the resulting queryset.
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.
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.
Using only()
and omitting a field requested using select_related()
is
an error as well. On the other hand, invoking only()
without any arguments,
will return every field (including annotations) fetched by the queryset.
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.
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.
You can't use select_for_update()
on nullable relations:
>>> 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
To avoid that restriction, you can exclude null objects if you don't care about them:
>>> 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
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
.
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 (&
)¶
Combines two QuerySet
s using the SQL AND
operator in a manner similar
to chaining filters.
Berikut adalah setara:
Model.objects.filter(x=1) & Model.objects.filter(y=2)
Model.objects.filter(x=1).filter(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 (^
)¶
Combines two QuerySet
s using the SQL XOR
operator. A XOR
expression matches rows that are matched by an odd number of operands.
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=MOD(
(CASE WHEN x THEN 1 ELSE 0 END) +
(CASE WHEN y THEN 1 ELSE 0 END) +
...
(CASE WHEN z THEN 1 ELSE 0 END),
2
)
In older versions, on databases without native support for the SQL
XOR
operator, XOR
returned rows that were matched by exactly
one operand. The previous behavior was not consistent with MySQL,
MariaDB, and Python behavior.
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.
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.")
create()
¶
- create(**kwargs)¶
- acreate(**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.
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.
Finally, a word on using get_or_create()
in Django views. Please make sure
to use it only in POST
requests unless you have a good reason not to.
GET
requests shouldn't have any effect on data. Instead, use POST
whenever a request to a page has a side effect on your data. For more, see
Safe methods in the HTTP spec.
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)
You can use get_or_create()
through Book's chapters field, but it only
fetches inside the context of that book:
>>> 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.
update_or_create()
¶
- update_or_create(defaults=None, create_defaults=None, **kwargs)¶
- aupdate_or_create(defaults=None, 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. Both create_defaults
and defaults
are
dictionaries of (field, value) pairs. The values in both create_defaults
and defaults
can be callables. defaults
is used to update the object
while create_defaults
are used for the create operation. If
create_defaults
is not supplied, defaults
will be used for the create
operation.
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"}
create_defaults = {"first_name": "Bob", "birthday": date(1940, 10, 9)}
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(create_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"},
create_defaults={"first_name": "Bob", "birthday": date(1940, 10, 9)},
)
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.
The create_defaults
argument 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 sinyalpre_save
danpost_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
andignore_conflicts
is False, the primary key attribute can only be retrieved on certain databases (currently PostgreSQL, MariaDB, 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 evaluatesobjs
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), 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
parameter disables setting the primary key on
each model instance (if the database normally supports it).
In older versions, enabling the update_conflicts
parameter prevented
setting the primary key on each model instance.
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.
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
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 thepre_save
andpost_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.
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.
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.
Example:
>>> 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.
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.
Support for aiterator()
with previous calls to prefetch_related()
was added.
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.
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")
earliest()
¶
- earliest(*fields)¶
- aearliest(*fields)¶
Asynchronous version: aearliest()
Bekerja sebalinya seperti latest()
kecuali arah dirubah.
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
last()
¶
- last()¶
- alast()¶
Asynchronous version: alast()
Bekerja seperti first()
, tetapi mengembalikan obyek terakhir dalam queryset.
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.
For example, when you are working with blog entries, you may want to know the number of authors that have contributed blog entries:
>>> from django.db.models import Count
>>> Blog.objects.aggregate(Count("entry"))
{'entry__count': 16}
By using a keyword argument to specify the aggregate function, you can control the name of the aggregation value that is returned:
>>> 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.
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.
contains()
¶
- contains(obj)¶
- acontains(obj)¶
Asynchronous version: acontains()
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.
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).
For example, to turn comments off for all blog entries published in 2010, you could do this:
>>> Entry.objects.filter(pub_date__year=2010).update(comments_on=False)
(Ini menganggap model Entry
mempunyai bidang pub_date
dan comments_on
.)
You can update multiple fields — there's no limit on how many. For example,
here we update the comments_on
and headline
fields:
>>> 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!
Filtering based on related fields is still possible, though:
>>> 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.
The update()
method returns the number of affected rows:
>>> 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()
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.
For example, to delete all the entries in a particular blog:
>>> 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.
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.
For example, when using 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.
Some databases accept flags that can return more information about the query. Pass these flags as keyword arguments. For example, when using 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.
Support for the generic_plan
option on PostgreSQL 16+ 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
.
Empty querysets or groups
Aggregation functions return None
when used with an empty QuerySet
or group. For example, the Sum
aggregation function returns None
instead of 0
if the QuerySet
contains no entries or for any empty
group in a non-empty QuerySet
. To return another value instead, define
the default
argument. Count
is an exception to this behavior; it
returns 0
if the QuerySet
is empty since 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
¶
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)[sumber]¶
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 isint
, otherwise same as input field, oroutput_field
if supplied. If the queryset or grouping is empty,default
is returned.
- distinct¶
Optional. If
distinct=True
,Avg
returns the mean value of unique values. This is the SQL equivalent ofAVG(DISTINCT <field>)
. The default value isFalse
.
Count
¶
- class Count(expression, distinct=False, filter=None, **extra)[sumber]¶
Returns the number of objects that are related through the provided expression.
Count('*')
is equivalent to the SQLCOUNT(*)
expression.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 ofCOUNT(DISTINCT <field>)
. The default value isFalse
.
Catatan
The
default
argument is not supported.
Max
¶
Min
¶
StdDev
¶
- class StdDev(expression, output_field=None, sample=False, filter=None, default=None, **extra)[sumber]¶
Mengembalikan selisih standar dari data dalam pernyataan yang disediakan.
Nama lain awalan:
1__stddev
Return type:
float
if input isint
, otherwise same as input field, oroutput_field
if supplied. If the queryset or grouping is empty,default
is returned.
- sample¶
Optional. By default,
StdDev
returns the population standard deviation. However, ifsample=True
, the return value will be the sample standard deviation.
Sum
¶
- class Sum(expression, output_field=None, distinct=False, filter=None, default=None, **extra)[sumber]¶
Menghitung jumlah dari semua nilai dari pernyataan yang diberikan.
Nama lain awalan:
1__sum
Return type: same as input field, or
output_field
if supplied. If the queryset or grouping is empty,default
is returned.
- distinct¶
Optional. If
distinct=True
,Sum
returns the sum of unique values. This is the SQL equivalent ofSUM(DISTINCT <field>)
. The default value isFalse
.
Variance
¶
- class Variance(expression, output_field=None, sample=False, filter=None, default=None, **extra)[sumber]¶
Mengembalikan beragam dari data di pernyataan yang disediakan.
Nama lain awalan:
1__variance
Return type:
float
if input isint
, otherwise same as input field, oroutput_field
if supplied. If the queryset or grouping is empty,default
is returned.
- sample¶
Optional. By default,
Variance
returns the population variance. However, ifsample=True
, the return value will be the sample variance.