Acuan contoh model¶
This document describes the details of the Model
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 Weblog models presented in the database query guide.
membuat obyek¶
To create a new instance of a model, just instantiate it like any other Python class:
The keyword arguments are simply the names of the fields you've defined on your
model. Note that instantiating a model in no way touches your database; for
that, you need to save()
.
Catatan
You may be tempted to customize the model by overriding the __init__
method. If you do so, however, take care not to change the calling
signature as any change may prevent the model instance from being saved.
Rather than overriding __init__
, try using one of these approaches:
Add a classmethod on the model class:
from django.db import models class Book(models.Model): title = models.CharField(max_length=100) @classmethod def create(cls, title): book = cls(title=title) # do something with the book return book book = Book.create("Pride and Prejudice")
Tambah sebuat metode pada pengelola penyesuaian (biasanya disukai):
class BookManager(models.Manager): def create_book(self, title): book = self.create(title=title) # do something with the book return book class Book(models.Model): title = models.CharField(max_length=100) objects = BookManager() book = Book.objects.create_book("Pride and Prejudice")
Menyesuaikan memuat model¶
The from_db()
method can be used to customize model instance creation
when loading from the database.
The db
argument contains the database alias for the database the model
is loaded from, field_names
contains the names of all loaded fields, and
values
contains the loaded values for each field in field_names
. The
field_names
are in the same order as the values
. If all of the model's
fields are present, then values
are guaranteed to be in the order
__init__()
expects them. That is, the instance can be created by
cls(*values)
. If any fields are deferred, they won't appear in
field_names
. In that case, assign a value of django.db.models.DEFERRED
to each of the missing fields.
In addition to creating the new model, the from_db()
method must set the
adding
and db
flags in the new instance's _state
attribute.
Below is an example showing how to record the initial values of fields that are loaded from the database:
from django.db.models import DEFERRED
@classmethod
def from_db(cls, db, field_names, values):
# Default implementation of from_db() (subject to change and could
# be replaced with super()).
if len(values) != len(cls._meta.concrete_fields):
values = list(values)
values.reverse()
values = [
values.pop() if f.attname in field_names else DEFERRED
for f in cls._meta.concrete_fields
]
instance = cls(*values)
instance._state.adding = False
instance._state.db = db
# customization to store the original field values on the instance
instance._loaded_values = dict(zip(field_names, values))
return instance
def save(self, *args, **kwargs):
# Check how the current values differ from ._loaded_values. For example,
# prevent changing the creator_id of the model. (This example doesn't
# support cases where 'creator_id' is deferred).
if not self._state.adding and (
self.creator_id != self._loaded_values['creator_id']):
raise ValueError("Updating the value of creator isn't allowed")
super().save(*args, **kwargs)
The example above shows a full from_db()
implementation to clarify how that
is done. In this case it would of course be possible to just use super()
call
in the from_db()
method.
Menyegarkan kembali obyek dari basisdata¶
Jika anda menghapus sebuah bidang dari sebuah contoh model, mengakses itu kembali memuat kembali nilai dari basisdata:
>>> obj = MyModel.objects.first()
>>> del obj.field
>>> obj.field # Loads the field from the database
If you need to reload a model's values from the database, you can use the
refresh_from_db()
method. When this method is called without arguments the
following is done:
- All non-deferred fields of the model are updated to the values currently present in the database.
- Any cached relations are cleared from the reloaded instance.
Hanya bidang-bidang dari model dimuat kembali dari basisdata. Nilai-nilai bergantung-basisdata lain seperti keterangan tidak dimuat kembali. Atribut @cached_property
apapun tidak dibersihkan juga.
The reloading happens from the database the instance was loaded from, or from
the default database if the instance wasn't loaded from the database. The
using
argument can be used to force the database used for reloading.
Itu memungkinkan untuk memaksa kumpulan dari bidang-bidang dimuat menggunakan argumen field
.
Sebagai contoh, untuk mencoba bahwa sebuah panggilan update()
dihasilkan dalam pembaharuan diharapkan, anda dapat menulis sebuah percobaan miriip seperti ini:
def test_update_result(self):
obj = MyModel.objects.create(val=1)
MyModel.objects.filter(pk=obj.pk).update(val=F('val') + 1)
# At this point obj.val is still 1, but the value in the database
# was updated to 2. The object's updated value needs to be reloaded
# from the database.
obj.refresh_from_db()
self.assertEqual(obj.val, 2)
Catat bahwa ketika bidang ditangguhkan diakses, memuat nilai bidang tertangguh terjadi melalui metode ini. dengan demikian itu adalah mungkin untuk menyesuaian cara penangguhan memuat terjadi. Contoh dibawah menunjukkan bagaimana satu dapat dimuat kembali dari bidang-bidang instance ketika bidang tertangguhkan dimuat:
class ExampleModel(models.Model):
def refresh_from_db(self, using=None, fields=None, **kwargs):
# fields contains the name of the deferred field to be
# loaded.
if fields is not None:
fields = set(fields)
deferred_fields = self.get_deferred_fields()
# If any deferred field is going to be loaded
if fields.intersection(deferred_fields):
# then load all of them
fields = fields.union(deferred_fields)
super().refresh_from_db(using, fields, **kwargs)
A helper method that returns a set containing the attribute names of all those fields that are currently deferred for this model.
Mensahkan obyek-obyek¶
Ada tidak langkah terlibat dalam mengesahkan sebuah model:
- Mensahkan bidang-bidang model -
Model.clean_fields()
- Mensahkan model sebagai sebuah keseluruhan -
Model.clean_fields()
- Mensahkan keunikan bidang -
Model.validate_unique()
Semua tiga langkah dilakukan ketika anda memanggil metode full_clean()
model.
Ketika anda menggunakan ModelForm
, panggilan pada is_valid()
akan melakukan langkah-langkah pengesahan ini untuk semua bidang yang disertakan pada formulir. Lihat ModelForm documentation ` untuk informasi lebih. Anda hanya butuh memanggil metode :meth:`~Model.full_clean() model jika anda berencana menangani kesalahan pengesahan anda sendiri, atau jika anda telah tidak menyertakan bidang-bidang dari ModelForm
yang membutuhkan pengesahan.
Metode ini memanggil Model.clean_fields()
, Model.clean()
, dan Model.validate_unique()
(jika validate_unique
adalah True
), dalam urutan itu dan memunculkan ValidationError
yang mempunyai atribut message_dict
mengandung kesalahan dari semua tiga tingkatan.
Argumen exclude
pilihan dapat digunakan untuk menyediakan daftar dari nama-nama bidang yang dapat tidak disertakan dari pengesahan dan pembersihan. ModelForm
menggunakan argumen ini untuk tidak menyertakan bidang-bidang yang tidak hadir pada formulir anda dari menjadi disahkan sejak kesalahan apapun muncul tidak dapat diperbaiki oleh pengguna.
Catata bahwa full_clean()
tidak akan dipanggil otomatis ketika anda memanggil metode save()
model anda. Anda akan butuh memanggil itu secara manual ketika anda ingin menjalankan pengesahan model satu-langkah untuk anda sendiri secara manual model dibuat. Sebagai contoh:
from django.core.exceptions import ValidationError
try:
article.full_clean()
except ValidationError as e:
# Do something based on the errors contained in e.message_dict.
# Display them to a user, or handle them programmatically.
pass
Langkah pertama full_clean()
lakukan adalah membersihkan setiap bidang masing-masing.
Metode ini akan mensahkan semua bidang pada model anda. Pilihan argumen exclude
membuat anda menyediakan daftar dari nama-nama bidang untuk tidak disertakan dari pengesahan. itu akan memunculkan ValidationError
jika bidang apapun gagal mensahkan.
Langkah kedua full_clean()
lakukan adalah memanggil Model.clean()
. Metode ini harus ditimpa untuk melakukan penyesuaian keabsahan pada model anda.
Metode ini harus digunakan untuk menyediakan pengesahan model penyesuaian, dan untuk merubah atribut pada model anda jika diinginkan. Sebagai contoh, anda dapat menggunakan itu secara otomatis menyediakan nilai untuk bidang, atau melakukan pengesahan yang membutuhkan akses pada lebih dari bidang tunggal:
import datetime
from django.core.exceptions import ValidationError
from django.db import models
from django.utils.translation import gettext_lazy as _
class Article(models.Model):
...
def clean(self):
# Don't allow draft entries to have a pub_date.
if self.status == 'draft' and self.pub_date is not None:
raise ValidationError(_('Draft entries may not have a publication date.'))
# Set the pub_date for published items if it hasn't been set already.
if self.status == 'published' and self.pub_date is None:
self.pub_date = datetime.date.today()
Catat, bagaimanapun, itu seeprti Model.full_clean()
, sebuah metode clean()
model tidak dipanggil ketika anda memanggil metode save()
model anda.
Dalam contoh diatas, pengecualian ValidationError
dimunculkan oleh Model.clean()
telah dipakai dengan string, sehingga itu akan tetap disimpan dalam kunci dictionary khusus, NON_FIELD_ERRORS
. Kunci ini digunakan untuk kesalahan-kesalahan yang diikat pada keseluruhan model daripada bidang tertentu:
from django.core.exceptions import NON_FIELD_ERRORS, ValidationError
try:
article.full_clean()
except ValidationError as e:
non_field_errors = e.message_dict[NON_FIELD_ERRORS]
Untuk memberikan pengecualian pada bidang khusus, pemakaian ValidationError
dengan sebuah dictionary, dimana kunci-kunci adalah nama-nama bidang. Kami dapat memperbaharui contoh sebelumnya untuk memberikan kesalahan pada bidang pub_date
:
class Article(models.Model):
...
def clean(self):
# Don't allow draft entries to have a pub_date.
if self.status == 'draft' and self.pub_date is not None:
raise ValidationError({'pub_date': _('Draft entries may not have a publication date.')})
...
Jika anda mengetahui kesalahan dalam banyak bidang selama Model.clean()
, anda dapat juga melewatkan nama-nama bidang pemetaan dictionary pada kesalahan:
raise ValidationError({
'title': ValidationError(_('Missing title.'), code='required'),
'pub_date': ValidationError(_('Invalid date.'), code='invalid'),
})
Akhirnya, full_clean()
akan memeriksa batsan unik pada model.
How to raise field-specific validation errors if those fields don't appear in a ModelForm
Anda tidak dapat memunculkan kesalahan pengesana dalam Model.clean()
untuk bidang yang tidak muncul dalam sebuah formulir model (formulir mungkin membatasi bidang-bidangnya menggunakan Meta.fields
atau Meta.exclude
). Melakukan itu akan memunculkan ValueError
karena kesalahan pengesahan tidak dapat dikaitkan dengan bidang tidak disertakan.
Untuk bekerja pada pilihan ini, daripada menimpa Model.clean_fields()
ketika itu menerima daftar dari bidang-bidang yang tidak disertakan dari pengesahan. Sebagai contoh:
class Article(models.Model):
...
def clean_fields(self, exclude=None):
super().clean_fields(exclude=exclude)
if self.status == 'draft' and self.pub_date is not None:
if exclude and 'status' in exclude:
raise ValidationError(
_('Draft entries may not have a publication date.')
)
else:
raise ValidationError({
'status': _(
'Set status to draft if there is not a '
'publication date.'
),
})
Metode ini mirip pada clean_fields()
, tetapi mensahkan semua batasan-batasan unik pada model anda daripada nilai bidang tersendiri. Argumen exclude
pilihan mengizinkan anda menyediakan daftar dari nama-nama bidang untuk tidak menyertakan dari pengesahan. Itu akan memunculkan ValidationError
jika bidang apapun gagal pengesahan.
Catat bahwa jika anda menyediakan sebuah argumen exclude
pada validate_unique()
, batasan unique_together
apapun melibatkan satu dari bidang-bidang anda sediakan tidak akan diperiksa.
Menyimpan obyek¶
Untuk menyimpan sebuah obyek kembali ke basisdata, panggil save()
:
-
Model.
save
(force_insert=False, force_update=False, using=DEFAULT_DB_ALIAS, update_fields=None)[sumber]¶
If you want customized saving behavior, you can override this save()
method. See Menimpa metode model yang detetapkan for more details.
The model save process also has some subtleties; see the sections below.
Peningkatan-otomatis primary key¶
Jika sebuah model mempunyai AutoField
— sebuah primary key menaik-otomatis - kemudian nilai menaik-otomatis akan dihitung dan disimpan sebagai sebuah atribut pada obyek anda pertama kali anda memanggil save()
:
>>> b2 = Blog(name='Cheddar Talk', tagline='Thoughts on cheese.')
>>> b2.id # Returns None, because b2 doesn't have an ID yet.
>>> b2.save()
>>> b2.id # Returns the ID of your new object.
There's no way to tell what the value of an ID will be before you call
save()
, because that value is calculated by your database, not by Django.
Untuk kenyamanan, setiap model memiliki sebuah AutoField
bernama id
secara awalan meskipun anda secara tegas menentukan primary_key=True
pada bidang dalam model anda. Lihat dokumentasi untuk AutoField
untuk rincian lebih.
Sifat pk
¶
-
Model.
pk
¶
Meskipun apakah anda menentukan bidang primary key anda sendiri, atau membiarkan Django mendukung satu untuk anda, setiap model akan memiliki sifat dipanggil pk
. Itu berperilaku seperti atribut biasa pada model, tetapi sebenarnya sebuah nama lain untuk atribut mana saja adalah bidang primary key untuk model. Anda dapat membaca dan mensetel nilai ini, sama seperti anda untuk atribut lain, dan itu akan memperbaharui bidang benar dalam model.
Explicitly specifying auto-primary-key values¶
If a model has an AutoField
but you want to define a
new object's ID explicitly when saving, just define it explicitly before
saving, rather than relying on the auto-assignment of the ID:
>>> b3 = Blog(id=3, name='Cheddar Talk', tagline='Thoughts on cheese.')
>>> b3.id # Returns 3.
>>> b3.save()
>>> b3.id # Returns 3.
If you assign auto-primary-key values manually, make sure not to use an already-existing primary-key value! If you create a new object with an explicit primary-key value that already exists in the database, Django will assume you're changing the existing record rather than creating a new one.
Diberikan contoh blog 'Cheddar Talk'
diatas, contoh ini akan menimpa rekaman sebelumnya di basisdata:
b4 = Blog(id=3, name='Not Cheddar', tagline='Anything but cheese.')
b4.save() # Overrides the previous blog with ID=3!
Lihat How Django knows to UPDATE vs. INSERT, dibawah, untuk alasan ini terjadi.
Explicitly specifying auto-primary-key values is mostly useful for bulk-saving objects, when you're confident you won't have primary-key collision.
If you're using PostgreSQL, the sequence associated with the primary key might need to be updated; see Manual-menentukan nilai-nilai dari primary key peningkatan-otomatis.
Apa yang terjadi ketika anda menyimpan?¶
Ketika anda menyimpan sebuah obyek, Django melakukan langkah-langkah berikut:
Emit a pre-save signal. The
pre_save
signal is sent, allowing any functions listening for that signal to do something.Preprocess the data. Each field's
pre_save()
method is called to perform any automated data modification that's needed. For example, the date/time fields overridepre_save()
to implementauto_now_add
andauto_now
.Prepare the data for the database. Each field's
get_db_prep_save()
method is asked to provide its current value in a data type that can be written to the database.Most fields don't require data preparation. Simple data types, such as integers and strings, are 'ready to write' as a Python object. However, more complex data types often require some modification.
For example,
DateField
fields use a Pythondatetime
object to store data. Databases don't storedatetime
objects, so the field value must be converted into an ISO-compliant date string for insertion into the database.Insert the data into the database. The preprocessed, prepared data is composed into an SQL statement for insertion into the database.
Emit a post-save signal. The
post_save
signal is sent, allowing any functions listening for that signal to do something.
Bagaimana Django mengetahui untuk UPDATE lawan INSERT¶
You may have noticed Django database objects use the same save()
method
for creating and changing objects. Django abstracts the need to use INSERT
or UPDATE
SQL statements. Specifically, when you call save()
, Django
follows this algorithm:
- If the object's primary key attribute is set to a value that evaluates to
True
(i.e., a value other thanNone
or the empty string), Django executes anUPDATE
. - If the object's primary key attribute is not set or if the
UPDATE
didn't update anything (e.g. if primary key is set to a value that doesn't exist in the database), Django executes anINSERT
.
The one gotcha here is that you should be careful not to specify a primary-key value explicitly when saving new objects, if you cannot guarantee the primary-key value is unused. For more on this nuance, see Explicitly specifying auto-primary-key values above and Forcing an INSERT or UPDATE below.
In Django 1.5 and earlier, Django did a SELECT
when the primary key
attribute was set. If the SELECT
found a row, then Django did an UPDATE
,
otherwise it did an INSERT
. The old algorithm results in one more query in
the UPDATE
case. There are some rare cases where the database doesn't
report that a row was updated even if the database contains a row for the
object's primary key value. An example is the PostgreSQL ON UPDATE
trigger
which returns NULL
. In such cases it is possible to revert to the old
algorithm by setting the select_on_save
option to True
.
Memaksa sebuah INSERT atau UPDATE¶
In some rare circumstances, it's necessary to be able to force the
save()
method to perform an SQL INSERT
and not fall back to
doing an UPDATE
. Or vice-versa: update, if possible, but not insert a new
row. In these cases you can pass the force_insert=True
or
force_update=True
parameters to the save()
method.
Obviously, passing both parameters is an error: you cannot both insert and
update at the same time!
It should be very rare that you'll need to use these parameters. Django will almost always do the right thing and trying to override that will lead to errors that are difficult to track down. This feature is for advanced use only.
Menggunakan update_fields
akan memaksa sebuah perbaharuan mirip pada force_update
.
Memperbaharui atribut berdasarkan pada bidang yang ada¶
Sometimes you'll need to perform a simple arithmetic task on a field, such as incrementing or decrementing the current value. The obvious way to achieve this is to do something like:
>>> product = Product.objects.get(name='Venezuelan Beaver Cheese')
>>> product.number_sold += 1
>>> product.save()
If the old number_sold
value retrieved from the database was 10, then
the value of 11 will be written back to the database.
The process can be made robust, avoiding a race condition, as well as slightly faster by expressing
the update relative to the original field value, rather than as an explicit
assignment of a new value. Django provides F expressions
for performing this kind of relative update. Using
F expressions
, the previous example is expressed
as:
>>> from django.db.models import F
>>> product = Product.objects.get(name='Venezuelan Beaver Cheese')
>>> product.number_sold = F('number_sold') + 1
>>> product.save()
For more details, see the documentation on F expressions
and their use in update queries.
Menentukan bidang-bidang mana yang disimpan¶
If save()
is passed a list of field names in keyword argument
update_fields
, only the fields named in that list will be updated.
This may be desirable if you want to update just one or a few fields on
an object. There will be a slight performance benefit from preventing
all of the model fields from being updated in the database. For example:
product.name = 'Name changed again'
product.save(update_fields=['name'])
The update_fields
argument can be any iterable containing strings. An
empty update_fields
iterable will skip the save. A value of None will
perform an update on all fields.
Menentukan update_fields
akan memaksa sebuah pembaharuan.
When saving a model fetched through deferred model loading
(only()
or
defer()
) only the fields loaded
from the DB will get updated. In effect there is an automatic
update_fields
in this case. If you assign or change any deferred field
value, the field will be added to the updated fields.
menghapus obyek¶
Issues an SQL DELETE
for the object. This only deletes the object in the
database; the Python instance will still exist and will still have data in
its fields. This method returns the number of objects deleted and a dictionary
with the number of deletions per object type.
Untuk rincian lebih, termasuk bagaimana menghapus obyek-obyek dalam kumpulan, lihat menghapus obyek.
Jika anda ingin menyesuaikan perilaku hapus, anda dapat menimpa metode delete()
. Lihat Menimpa metode model yang detetapkan untuk lebih rinci.
Sometimes with multi-table inheritance you may
want to delete only a child model's data. Specifying keep_parents=True
will
keep the parent model's data.
Pickling objects¶
When you pickle
a model, its current state is pickled. When you unpickle
it, it'll contain the model instance at the moment it was pickled, rather than
the data that's currently in the database.
Other model instance methods¶
A few object methods have special purposes.
__str__()
¶
The __str__()
method is called whenever you call str()
on an object.
Django uses str(obj)
in a number of places. Most notably, to display an
object in the Django admin site and as the value inserted into a template when
it displays an object. Thus, you should always return a nice, human-readable
representation of the model from the __str__()
method.
Sebagai contoh:
from django.db import models
class Person(models.Model):
first_name = models.CharField(max_length=50)
last_name = models.CharField(max_length=50)
def __str__(self):
return '%s %s' % (self.first_name, self.last_name)
__eq__()
¶
The equality method is defined such that instances with the same primary
key value and the same concrete class are considered equal, except that
instances with a primary key value of None
aren't equal to anything except
themselves. For proxy models, concrete class is defined as the model's first
non-proxy parent; for all other models it's simply the model's class.
Sebagai contoh:
from django.db import models
class MyModel(models.Model):
id = models.AutoField(primary_key=True)
class MyProxyModel(MyModel):
class Meta:
proxy = True
class MultitableInherited(MyModel):
pass
# Primary keys compared
MyModel(id=1) == MyModel(id=1)
MyModel(id=1) != MyModel(id=2)
# Primary keys are None
MyModel(id=None) != MyModel(id=None)
# Same instance
instance = MyModel(id=None)
instance == instance
# Proxy model
MyModel(id=1) == MyProxyModel(id=1)
# Multi-table inheritance
MyModel(id=1) != MultitableInherited(id=1)
__hash__()
¶
The __hash__()
method is based on the instance's primary key value. It
is effectively hash(obj.pk)
. If the instance doesn't have a primary key
value then a TypeError
will be raised (otherwise the __hash__()
method would return different values before and after the instance is
saved, but changing the __hash__()
value of an instance is
forbidden in Python.
get_absolute_url()
¶
-
Model.
get_absolute_url
()¶
Define a get_absolute_url()
method to tell Django how to calculate the
canonical URL for an object. To callers, this method should appear to return a
string that can be used to refer to the object over HTTP.
Sebagai contoh:
def get_absolute_url(self):
return "/people/%i/" % self.id
While this code is correct and simple, it may not be the most portable way to
to write this kind of method. The reverse()
function is
usually the best approach.
Sebagai contoh:
def get_absolute_url(self):
from django.urls import reverse
return reverse('people.views.details', args=[str(self.id)])
One place Django uses get_absolute_url()
is in the admin app. If an object
defines this method, the object-editing page will have a "View on site" link
that will jump you directly to the object's public view, as given by
get_absolute_url()
.
Similarly, a couple of other bits of Django, such as the syndication feed
framework, use get_absolute_url()
when it is
defined. If it makes sense for your model's instances to each have a unique
URL, you should define get_absolute_url()
.
Peringatan
You should avoid building the URL from unvalidated user input, in order to reduce possibilities of link or redirect poisoning:
def get_absolute_url(self):
return '/%s/' % self.name
If self.name
is '/example.com'
this returns '//example.com/'
which, in turn, is a valid schema relative URL but not the expected
'/%2Fexample.com/'
.
It's good practice to use get_absolute_url()
in templates, instead of
hard-coding your objects' URLs. For example, this template code is bad:
<!-- BAD template code. Avoid! -->
<a href="/people/{{ object.id }}/">{{ object.name }}</a>
Kode cetakan ini jauh lebih baik:
<a href="{{ object.get_absolute_url }}">{{ object.name }}</a>
The logic here is that if you change the URL structure of your objects, even
for something simple such as correcting a spelling error, you don't want to
have to track down every place that the URL might be created. Specify it once,
in get_absolute_url()
and have all your other code call that one place.
Catatan
The string you return from get_absolute_url()
must contain only
ASCII characters (required by the URI specification, RFC 2396#section-2)
and be URL-encoded, if necessary.
Code and templates calling get_absolute_url()
should be able to use the
result directly without any further processing. You may wish to use the
django.utils.encoding.iri_to_uri()
function to help with this if you
are using strings containing characters outside the ASCII range.
Extra instance methods¶
In addition to save()
, delete()
, a model object
might have some of the following methods:
-
Model.
get_FOO_display
()¶
For every field that has choices
set, the
object will have a get_FOO_display()
method, where FOO
is the name of
the field. This method returns the "human-readable" value of the field.
Sebagai contoh:
from django.db import models
class Person(models.Model):
SHIRT_SIZES = (
('S', 'Small'),
('M', 'Medium'),
('L', 'Large'),
)
name = models.CharField(max_length=60)
shirt_size = models.CharField(max_length=2, choices=SHIRT_SIZES)
>>> p = Person(name="Fred Flintstone", shirt_size="L")
>>> p.save()
>>> p.shirt_size
'L'
>>> p.get_shirt_size_display()
'Large'
-
Model.
get_next_by_FOO
(**kwargs)¶
-
Model.
get_previous_by_FOO
(**kwargs)¶
For every DateField
and
DateTimeField
that does not have null=True
, the object will have get_next_by_FOO()
and
get_previous_by_FOO()
methods, where FOO
is the name of the field. This
returns the next and previous object with respect to the date field, raising
a DoesNotExist
exception when appropriate.
Both of these methods will perform their queries using the default manager for the model. If you need to emulate filtering used by a custom manager, or want to perform one-off custom filtering, both methods also accept optional keyword arguments, which should be in the format described in Field lookups.
Note that in the case of identical date values, these methods will use the primary key as a tie-breaker. This guarantees that no records are skipped or duplicated. That also means you cannot use those methods on unsaved objects.
Atribut-atribut lain¶
DoesNotExist
¶
-
exception
Model.
DoesNotExist
¶ This exception is raised by the ORM in a couple places, for example by
QuerySet.get()
when an object is not found for the given query parameters.Django provides a
DoesNotExist
exception as an attribute of each model class to identify the class of object that could not be found and to allow you to catch a particular model class withtry/except
. The exception is a subclass ofdjango.core.exceptions.ObjectDoesNotExist
.