モデルインスタンスリファレンス

このドキュメントでは、Model API の詳細を説明しています。モデル と データベースクエリ ガイドにある説明を前提としていますので、このドキュメントを読む前にこの 2 つを読んでおいた方がよいでしょう。

このドキュメント全体を通して、データベースクエリガイド で使った Weblog モデル例 を使用します。

オブジェクトを作成する

モデルの新しいインスタンスを作成するには、他の Python クラスと同様に単にインスタンス化してください:

class Model(**kwargs)[ソース]

キーワード引数は、シンプルに、モデル内で定義したフィールドの名前です。モデルをインスタンス化しても、データベースには何もしないことに注意してください。データベースとやり取りするには save() を使う必要があります。

注釈

__init__ メソッドをオーバーライドしてモデルをカスタマイズする誘惑に駆られるかもしれません。その場合は、とは言え、あらゆる変更がモデルインスタンスを保存しないように、呼び出しシグネチャを変更しないように配慮してください。__init__ をオーバーライドするのではなく、以下のいずれかのアプローチを試してみてください:

  1. モデルクラスにクラスメソッドを追加する:

    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")

  2. 独自のマネジャーにメソッドを追加する (通常推奨される方法です):

    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")

モデルの読み込みをカスタマイズする

classmethod Model.from_db(db, field_names, values)[ソース]

from_db() メソッドを使うと、データベースから読み込むときのモデルインスタンス作成をカスタマイズすることができます。

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
        ]
    new = 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.

Changed in Django 1.10:

In older versions, you could check if all fields were loaded by consulting cls._deferred. This attribute is removed and django.db.models.DEFERRED is new.

Refreshing objects from database

If you delete a field from a model instance, accessing it again reloads the value from the database:

>>> obj = MyModel.objects.first()
>>> del obj.field
>>> obj.field  # Loads the field from the database
Changed in Django 1.10:

In older versions, accessing a deleted field raised AttributeError instead of reloading it.

Model.refresh_from_db(using=None, fields=None)[ソース]

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:

  1. All non-deferred fields of the model are updated to the values currently present in the database.
  2. The previously loaded related instances for which the relation’s value is no longer valid are removed from the reloaded instance. For example, if you have a foreign key from the reloaded instance to another model with name Author, then if obj.author_id != obj.author.id, obj.author will be thrown away, and when next accessed it will be reloaded with the value of obj.author_id.

Only fields of the model are reloaded from the database. Other database-dependent values such as annotations aren’t reloaded. Any @cached_property attributes aren’t cleared either.

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.

It is possible to force the set of fields to be loaded by using the fields argument.

For example, to test that an update() call resulted in the expected update, you could write a test similar to this:

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)

Note that when deferred fields are accessed, the loading of the deferred field’s value happens through this method. Thus it is possible to customize the way deferred loading happens. The example below shows how one can reload all of the instance’s fields when a deferred field is reloaded:

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(ExampleModel, self).refresh_from_db(using, fields, **kwargs)
Model.get_deferred_fields()[ソース]

A helper method that returns a set containing the attribute names of all those fields that are currently deferred for this model.

オブジェクトを検証 (バリデーション) する

モデルのバリデーションには 3 つのステップがあります:

  1. モデルフィールドを検証する - Model.clean_fields()

  2. モデル全体を検証する - Model.clean()

  3. フィールドの一意性を検証する - Model.validate_unique()

モデルの full_clean() メソッドを使うと、3 つ全てのステップが実行されます。

ModelForm を使っているとき、is_valid() の呼び出しは、フォーム上に含まれる全てのフィールドに対して、これらの検証ステップを実行します。詳しくは ModelForm ドキュメント を参照してください。検証エラーを自分でコントロールしたい場合、もしくは ModelForm から検証を必要とするフィールドを除外した場合は、モデルの full_clean() メソッドだけを呼び出す必要があります。

Model.full_clean(exclude=None, validate_unique=True)[ソース]

このメソッドは、Model.clean_fields()、Model.clean()、そして Model.validate_unique() (validate_unique が True の場合) をこの順序で呼び出し、3 つ全てのステージでのエラーを含む message_dict 属性を持つ ValidationError を投げます。

省略可能な exclude 引数は、検証とクリーニングから除外するフィールド名のリストに使います。ModelForm は、この引数をフォーム上に存在しないフィールドを検証対象から除外するために使います。これは、エラーが発生してもユーザーによって修正できないからです。

モデルの save() メソッドを呼んだときでも full_clean() は自動的に 呼ばれない ことに注意してください。手動で作ったモデルに対して 1 ステップでモデル検証をしたいときには、手動で呼び出す必要があります。例えば:

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

full_clean() が行う最初のステップは、個々のフィールドをそれぞれクリーニングすることです。

Model.clean_fields(exclude=None)[ソース]

This method will validate all fields on your model. The optional exclude argument lets you provide a list of field names to exclude from validation. It will raise a ValidationError if any fields fail validation.

The second step full_clean() performs is to call Model.clean(). This method should be overridden to perform custom validation on your model.

Model.clean()[ソース]

This method should be used to provide custom model validation, and to modify attributes on your model if desired. For instance, you could use it to automatically provide a value for a field, or to do validation that requires access to more than a single field:

import datetime
from django.core.exceptions import ValidationError
from django.db import models
from django.utils.translation import ugettext_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()

Note, however, that like Model.full_clean(), a model’s clean() method is not invoked when you call your model’s save() method.

In the above example, the ValidationError exception raised by Model.clean() was instantiated with a string, so it will be stored in a special error dictionary key, NON_FIELD_ERRORS. This key is used for errors that are tied to the entire model instead of to a specific field:

from django.core.exceptions import ValidationError, NON_FIELD_ERRORS
try:
    article.full_clean()
except ValidationError as e:
    non_field_errors = e.message_dict[NON_FIELD_ERRORS]

To assign exceptions to a specific field, instantiate the ValidationError with a dictionary, where the keys are the field names. We could update the previous example to assign the error to the pub_date field:

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.')})
        ...

If you detect errors in multiple fields during Model.clean(), you can also pass a dictionary mapping field names to errors:

raise ValidationError({
    'title': ValidationError(_('Missing title.'), code='required'),
    'pub_date': ValidationError(_('Invalid date.'), code='invalid'),
})

Finally, full_clean() will check any unique constraints on your model.

Model.validate_unique(exclude=None)[ソース]

This method is similar to clean_fields(), but validates all uniqueness constraints on your model instead of individual field values. The optional exclude argument allows you to provide a list of field names to exclude from validation. It will raise a ValidationError if any fields fail validation.

Note that if you provide an exclude argument to validate_unique(), any unique_together constraint involving one of the fields you provided will not be checked.

オブジェクトを保存する

データベースにオブジェクトを保存し直すには、save() を呼び出します:

Model.save(force_insert=False, force_update=False, using=DEFAULT_DB_ALIAS, update_fields=None)[ソース]

保存の動作をカスタマイズしたい場合は、save() メソッドをオーバーライドできます。詳しくは Overriding predefined model methods を参照してください。

モデル保存のプロセスには、いくつかの細かな注意点もあります; 以下の各セクションを参照してください。

自動インクリメントのプライマリキー

If a model has an AutoField — an auto-incrementing primary key — then that auto-incremented value will be calculated and saved as an attribute on your object the first time you call save():

>>> b2 = Blog(name='Cheddar Talk', tagline='Thoughts on cheese.')
>>> b2.id     # Returns None, because b 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.

For convenience, each model has an AutoField named id by default unless you explicitly specify primary_key=True on a field in your model. See the documentation for AutoField for more details.

pk プロパティ

Model.pk

Regardless of whether you define a primary key field yourself, or let Django supply one for you, each model will have a property called pk. It behaves like a normal attribute on the model, but is actually an alias for whichever attribute is the primary key field for the model. You can read and set this value, just as you would for any other attribute, and it will update the correct field in the 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.

Given the above 'Cheddar Talk' blog example, this example would override the previous record in the database:

b4 = Blog(id=3, name='Not Cheddar', tagline='Anything but cheese.')
b4.save()  # Overrides the previous blog with ID=3!

See How Django knows to UPDATE vs. INSERT, below, for the reason this happens.

Explicitly specifying auto-primary-key values is mostly useful for bulk-saving objects, when you’re confident you won’t have primary-key collision.

What happens when you save?

When you save an object, Django performs the following steps:

  1. Emit a pre-save signal. The pre_save signal is sent, allowing any functions listening for that signal to do something.

  2. 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 override pre_save() to implement auto_now_add and auto_now.

  3. 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 Python datetime object to store data. Databases don’t store datetime objects, so the field value must be converted into an ISO-compliant date string for insertion into the database.

  4. Insert the data into the database. The preprocessed, prepared data is composed into an SQL statement for insertion into the database.

  5. Emit a post-save signal. The post_save signal is sent, allowing any functions listening for that signal to do something.

Django が UPDATE と INSERT を見分ける方法

Django データベースオブジェクトが、オブジェクトの作成と変更に同じ save() メソッドを使うことにお気づきかもしれません。Django は INSERT や UPDATE といった SQL ステートメントを抽象化しています。特に、save() を呼び出すとき、Django は以下のアルゴリズムを実行します:

  • オブジェクトのプライマリキー属性が True と評価される値にセットされている場合 (例えば None や空の文字列以外の場合)。Django は UPDATE を実行します。

  • オブジェクトのプライマリキー属性がセット されていない、もしくは UPDATE が何もアップデートしなかった場合、Django は INSERT を実行します。

ここで重要なのは、そのプライマリキーが使われていないことが保証できない場合、新しいオブジェクトを保存するときに明示的にプライマリキーの値を指定しないように気をつけることです。このニュアンスについては、上述の 自動プライマリキーの値を明示的に指定する と下記の INSERT や UPDATE を強制する を参照してください。

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.

INSERT や UPDATE を強制する

まれに、save() に対して SQL の INSERT を強制してフォールバックで UPDATE を実行しない (もしくは逆に: 可能ならば更新をして新しい行を追加しない) ように強制する必要があります。こうしたケースでは、force_insert=True や force_update=True パラメータを save() メソッドに渡すことができます。当然のことですが、両方のパラメータを渡すとエラーになります: 追加と更新を同時にはできません！

これらのパラメータを必要とするのは、まれであるべきです。Django は、ほとんどの場合で正しく処理を行い、オーバーライドしようとすると追跡が難しいエラーの原因となります。この機能は応用的な場合のみに使用してください。

update_fields を使うと force_update と同じように更新を強制します。

既存のフィールドに基づいて属性を更新する

現在の値をインクリメントまたはデクリメントするなど、フィールドで単純な算術タスクを実行する必要が生じることがあります。 これを達成する明白な方法は、以下のようにすることです:

>>> product = Product.objects.get(name='Venezuelan Beaver Cheese')
>>> product.number_sold += 1
>>> product.save()

データベースから取得した古い number_sold の値は 10 でした。そして、11 という値がデータベースに書き直されます。

このプロセスは、新しい値の明示的な割り当てではなく、元のフィールド値に対する更新を表現することによって、競合状態を回避しながら 堅牢にすることができます。 Djangoは、このような相対的な更新を行うための F expressions を提供しています。 F expressions を使用すると、上の例は以下のように表されます:

>>> from django.db.models import F
>>> product = Product.objects.get(name='Venezuelan Beaver Cheese')
>>> product.number_sold = F('number_sold') + 1
>>> product.save()

詳しくは、F expressions とこれに対する 更新クエリ内で使う を参照してください。

どのフィールドを保存するか指定する

save() にキーワード引数 update_fields 内でフィールドリストの名前が渡された場合、このリスト内で指定されたフィールドだけが更新されます。これは、オブジェクトの 1 つないし少数のフィールドだけを更新したい場合に望ましいでしょう。データベースで全てのモデルフィールドを更新しないようにすると、実行速度が少し向上します。例えば:

product.name = 'Name changed again'
product.save(update_fields=['name'])

update_fields 引数は文字列を含む任意のイテラブルです。空の update_fields イテラブルは保存をスキップします。None という値は全てのフィールドを更新します。

update_fields を指定すると更新を強制します。

遅延モデルローディング (only() や defer()) を通してフェッチされたモデルを保存するとき、DB からロードされたフィールドだけが更新されます。実際には、このケースでは自動的な update_fields が存在します。遅延フィールドの値を追加もしくは変更した場合、フィールドは更新されたフィールドに追加されます。

オブジェクトを削除する

Model.delete(using=DEFAULT_DB_ALIAS, keep_parents=False)[ソース]

オブジェクトに対して SQL DELETE を発行します。これはデータベースでオブジェクトの削除のみを行います; Python のインスタンスはなお存在しまたフィールド内にデータを持ち続けます。このメソッドは削除されるオブジェクトの数とオブジェクトタイプごとの削除の数のディクショナリを返します。

より詳しく (まとめてオブジェクトを削除する方法を含む) については、オブジェクトを削除する を参照してください。

独自の削除動作がほしいときは、delete() メソッドをオーバーライドすることができます。詳しくは Overriding predefined model methods を参照してください。

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.

Changed in Django 1.9:

The keep_parents parameter was added.

Changed in Django 1.9:

The return value describing the number of objects deleted was added.

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.

You can’t share pickles between versions

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

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

Other model instance methods

A few object methods have special purposes.

__str__()

Model.__str__()[ソース]

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.

For example:

from django.db import models
from django.utils.encoding import python_2_unicode_compatible

@python_2_unicode_compatible  # only if you need to support Python 2
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)

If you’d like compatibility with Python 2, you can decorate your model class with python_2_unicode_compatible() as shown above.

__eq__()

Model.__eq__()[ソース]

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.

For example:

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)
# Primay 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__()

Model.__hash__()[ソース]

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.

For example:

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.

For example:

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().

警告

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>

This template code is much better:

<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.

注釈

The string you return from get_absolute_url() must contain only ASCII characters (required by the URI specification, RFC 2396) 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 unicode strings containing characters outside the ASCII range at all.

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.

For example:

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.

Other attributes

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 with try/except. The exception is a subclass of django.core.exceptions.ObjectDoesNotExist.

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