django/docs/ref/contrib/contenttypes.txt

==========================
The contenttypes framework
==========================

.. module:: django.contrib.contenttypes
   :synopsis: Provides generic interface to installed models.

Django includes a :mod:`~django.contrib.contenttypes` application that can
track all of the models installed in your Django-powered project, providing a
high-level, generic interface for working with your models.

Overview
========

At the heart of the contenttypes application is the
:class:`~django.contrib.contenttypes.models.ContentType` model, which lives at
``django.contrib.contenttypes.models.ContentType``. Instances of
:class:`~django.contrib.contenttypes.models.ContentType` represent and store
information about the models installed in your project, and new instances of
:class:`~django.contrib.contenttypes.models.ContentType` are automatically
created whenever new models are installed.

Instances of :class:`~django.contrib.contenttypes.models.ContentType` have
methods for returning the model classes they represent and for querying objects
from those models. :class:`~django.contrib.contenttypes.models.ContentType`
also has a :ref:`custom manager <custom-managers>` that adds methods for
working with :class:`~django.contrib.contenttypes.models.ContentType` and for
obtaining instances of :class:`~django.contrib.contenttypes.models.ContentType`
for a particular model.

Relations between your models and
:class:`~django.contrib.contenttypes.models.ContentType` can also be used to
enable "generic" relationships between an instance of one of your
models and instances of any model you have installed.

Installing the contenttypes framework
=====================================

The contenttypes framework is included in the default
:setting:`INSTALLED_APPS` list created by ``django-admin.py startproject``,
but if you've removed it or if you manually set up your
:setting:`INSTALLED_APPS` list, you can enable it by adding
``'django.contrib.contenttypes'`` to your :setting:`INSTALLED_APPS` setting.

It's generally a good idea to have the contenttypes framework
installed; several of Django's other bundled applications require it:

* The admin application uses it to log the history of each object
  added or changed through the admin interface.

* Django's :mod:`authentication framework <django.contrib.auth>` uses it
  to tie user permissions to specific models.

* Django's comments system (:mod:`django.contrib.comments`) uses it to
  "attach" comments to any installed model.

.. currentmodule:: django.contrib.contenttypes.models

The ``ContentType`` model
=========================

.. class:: ContentType

    Each instance of :class:`~django.contrib.contenttypes.models.ContentType`
    has three fields which, taken together, uniquely describe an installed
    model:

    .. attribute:: app_label

        The name of the application the model is part of. This is taken from
        the :attr:`app_label` attribute of the model, and includes only the
        *last* part of the application's Python import path;
        "django.contrib.contenttypes", for example, becomes an
        :attr:`app_label` of "contenttypes".

    .. attribute:: model

        The name of the model class.

    .. attribute:: name

        The human-readable name of the model. This is taken from the
        :attr:`verbose_name <django.db.models.Field.verbose_name>`
        attribute of the model.

Let's look at an example to see how this works. If you already have
the :mod:`~django.contrib.contenttypes` application installed, and then add
:mod:`the sites application <django.contrib.sites>` to your
:setting:`INSTALLED_APPS` setting and run ``manage.py syncdb`` to install it,
the model :class:`django.contrib.sites.models.Site` will be installed into
your database. Along with it a new instance of
:class:`~django.contrib.contenttypes.models.ContentType` will be
created with the following values:

* :attr:`~django.contrib.contenttypes.models.ContentType.app_label`
  will be set to ``'sites'`` (the last part of the Python
  path "django.contrib.sites").

* :attr:`~django.contrib.contenttypes.models.ContentType.model`
  will be set to ``'site'``.

* :attr:`~django.contrib.contenttypes.models.ContentType.name`
  will be set to ``'site'``.

.. _the verbose_name attribute: ../model-api/#verbose_name

Methods on ``ContentType`` instances
====================================

Each :class:`~django.contrib.contenttypes.models.ContentType` instance has
methods that allow you to get from a
:class:`~django.contrib.contenttypes.models.ContentType` instance to the
model it represents, or to retrieve objects from that model:

.. method:: ContentType.get_object_for_this_type(**kwargs)

    Takes a set of valid :ref:`lookup arguments <field-lookups-intro>` for the
    model the :class:`~django.contrib.contenttypes.models.ContentType`
    represents, and does
    :meth:`a get() lookup <django.db.models.query.QuerySet.get>`
    on that model, returning the corresponding object.

.. method:: ContentType.model_class()

    Returns the model class represented by this
    :class:`~django.contrib.contenttypes.models.ContentType` instance.

For example, we could look up the
:class:`~django.contrib.contenttypes.models.ContentType` for the
:class:`~django.contrib.auth.models.User` model::

    >>> from django.contrib.contenttypes.models import ContentType
    >>> user_type = ContentType.objects.get(app_label="auth", model="user")
    >>> user_type
    <ContentType: user>

And then use it to query for a particular
:class:`~django.contrib.auth.models.User`, or to get access
to the ``User`` model class::

    >>> user_type.model_class()
    <class 'django.contrib.auth.models.User'>
    >>> user_type.get_object_for_this_type(username='Guido')
    <User: Guido>

Together,
:meth:`~django.contrib.contenttypes.models.ContentType.get_object_for_this_type`
and :meth:`~django.contrib.contenttypes.models.ContentType.model_class` enable
two extremely important use cases:

1. Using these methods, you can write high-level generic code that
   performs queries on any installed model -- instead of importing and
   using a single specific model class, you can pass an ``app_label`` and
   ``model`` into a
   :class:`~django.contrib.contenttypes.models.ContentType` lookup at
   runtime, and then work with the model class or retrieve objects from it.

2. You can relate another model to
   :class:`~django.contrib.contenttypes.models.ContentType` as a way of
   tying instances of it to particular model classes, and use these methods
   to get access to those model classes.

Several of Django's bundled applications make use of the latter technique.
For example,
:class:`the permissions system <django.contrib.auth.models.Permission>` in
Django's authentication framework uses a
:class:`~django.contrib.auth.models.Permission` model with a foreign
key to :class:`~django.contrib.contenttypes.models.ContentType`; this lets
:class:`~django.contrib.auth.models.Permission` represent concepts like
"can add blog entry" or "can delete news story".

The ``ContentTypeManager``
--------------------------

.. class:: ContentTypeManager

    :class:`~django.contrib.contenttypes.models.ContentType` also has a custom
    manager, :class:`~django.contrib.contenttypes.models.ContentTypeManager`,
    which adds the following methods:

    .. method:: clear_cache()

        Clears an internal cache used by
        :class:`~django.contrib.contenttypes.models.ContentType` to keep track
        of which models for which it has created
        :class:`~django.contrib.contenttypes.models.ContentType` instances. You
        probably won't ever need to call this method yourself; Django will call
        it automatically when it's needed.

    .. method:: get_for_model(model)

        Takes either a model class or an instance of a model, and returns the
        :class:`~django.contrib.contenttypes.models.ContentType` instance
        representing that model.

    .. method:: get_for_models(*models)

        Takes a variadic number of model classes, and returns a dictionary
        mapping the model classes to the
        :class:`~django.contrib.contenttypes.models.ContentType` instances
        representing them.

    .. method:: get_by_natural_key(app_label, model)

        Returns the :class:`~django.contrib.contenttypes.models.ContentType`
        instance uniquely identified by the given application label and model
        name. The primary purpose of this method is to allow
        :class:`~django.contrib.contenttypes.models.ContentType` objects to be
        referenced via a :ref:`natural key<topics-serialization-natural-keys>`
        during deserialization.

The :meth:`~ContentTypeManager.get_for_model()` method is especially
useful when you know you need to work with a
:class:`ContentType <django.contrib.contenttypes.models.ContentType>` but don't
want to go to the trouble of obtaining the model's metadata to perform a manual
lookup::

    >>> from django.contrib.auth.models import User
    >>> user_type = ContentType.objects.get_for_model(User)
    >>> user_type
    <ContentType: user>

.. module:: django.contrib.contenttypes.generic

.. _generic-relations:

Generic relations
=================

Adding a foreign key from one of your own models to
:class:`~django.contrib.contenttypes.models.ContentType` allows your model to
effectively tie itself to another model class, as in the example of the
:class:`~django.contrib.auth.models.Permission` model above. But it's possible
to go one step further and use
:class:`~django.contrib.contenttypes.models.ContentType` to enable truly
generic (sometimes called "polymorphic") relationships between models.

A simple example is a tagging system, which might look like this::

    from django.db import models
    from django.contrib.contenttypes.models import ContentType
    from django.contrib.contenttypes import generic

    class TaggedItem(models.Model):
        tag = models.SlugField()
        content_type = models.ForeignKey(ContentType)
        object_id = models.PositiveIntegerField()
        content_object = generic.GenericForeignKey('content_type', 'object_id')

        def __unicode__(self):
            return self.tag

A normal :class:`~django.db.models.ForeignKey` can only "point
to" one other model, which means that if the ``TaggedItem`` model used a
:class:`~django.db.models.ForeignKey` it would have to
choose one and only one model to store tags for. The contenttypes
application provides a special field type which
works around this and allows the relationship to be with any
model:

.. class:: GenericForeignKey

    There are three parts to setting up a
    :class:`~django.contrib.contenttypes.generic.GenericForeignKey`:

    1. Give your model a :class:`~django.db.models.ForeignKey`
       to :class:`~django.contrib.contenttypes.models.ContentType`.

    2. Give your model a field that can store primary key values from the
       models you'll be relating to. For most models, this means a
       :class:`~django.db.models.PositiveIntegerField`. The usual name
       for this field is "object_id".

    3. Give your model a
       :class:`~django.contrib.contenttypes.generic.GenericForeignKey`, and
       pass it the names of the two fields described above. If these fields
       are named "content_type" and "object_id", you can omit this -- those
       are the default field names
       :class:`~django.contrib.contenttypes.generic.GenericForeignKey` will
       look for.

.. admonition:: Primary key type compatibility

   The "object_id" field doesn't have to be the same type as the
   primary key fields on the related models, but their primary key values
   must be coercible to the same type as the "object_id" field by its
   :meth:`~django.db.models.Field.get_db_prep_value` method.

   For example, if you want to allow generic relations to models with either
   :class:`~django.db.models.IntegerField` or
   :class:`~django.db.models.CharField` primary key fields, you
   can use :class:`~django.db.models.CharField` for the
   "object_id" field on your model since integers can be coerced to
   strings by :meth:`~django.db.models.Field.get_db_prep_value`.

   For maximum flexibility you can use a
   :class:`~django.db.models.TextField` which doesn't have a
   maximum length defined, however this may incur significant performance
   penalties depending on your database backend.

   There is no one-size-fits-all solution for which field type is best. You
   should evaluate the models you expect to be pointing to and determine
   which solution will be most effective for your use case.

.. admonition:: Serializing references to ``ContentType`` objects

   If you're serializing data (for example, when generating
   :class:`~django.test.TestCase.fixtures`) from a model that implements
   generic relations, you should probably be using a natural key to uniquely
   identify related :class:`~django.contrib.contenttypes.models.ContentType`
   objects. See :ref:`natural keys<topics-serialization-natural-keys>` and
   :djadminopt:`dumpdata --natural <--natural>` for more information.

This will enable an API similar to the one used for a normal
:class:`~django.db.models.ForeignKey`;
each ``TaggedItem`` will have a ``content_object`` field that returns the
object it's related to, and you can also assign to that field or use it when
creating a ``TaggedItem``::

    >>> from django.contrib.auth.models import User
    >>> guido = User.objects.get(username='Guido')
    >>> t = TaggedItem(content_object=guido, tag='bdfl')
    >>> t.save()
    >>> t.content_object
    <User: Guido>

Due to the way :class:`~django.contrib.contenttypes.generic.GenericForeignKey`
is implemented, you cannot use such fields directly with filters (``filter()``
and ``exclude()``, for example) via the database API. Because a
:class:`~django.contrib.contenttypes.generic.GenericForeignKey` isn't a
normal field objects, these examples will *not* work::

    # This will fail
    >>> TaggedItem.objects.filter(content_object=guido)
    # This will also fail
    >>> TaggedItem.objects.get(content_object=guido)

Reverse generic relations
-------------------------

.. class:: GenericRelation

If you know which models you'll be using most often, you can also add
a "reverse" generic relationship to enable an additional API. For example::

    class Bookmark(models.Model):
        url = models.URLField()
        tags = generic.GenericRelation(TaggedItem)

``Bookmark`` instances will each have a ``tags`` attribute, which can
be used to retrieve their associated ``TaggedItems``::

    >>> b = Bookmark(url='http://www.djangoproject.com/')
    >>> b.save()
    >>> t1 = TaggedItem(content_object=b, tag='django')
    >>> t1.save()
    >>> t2 = TaggedItem(content_object=b, tag='python')
    >>> t2.save()
    >>> b.tags.all()
    [<TaggedItem: django>, <TaggedItem: python>]

Just as :class:`~django.contrib.contenttypes.generic.GenericForeignKey`
accepts the names of the content-type and object-ID fields as
arguments, so too does
:class:`~django.contrib.contenttypes.generic.GenericRelation`;
if the model which has the generic foreign key is using non-default names
for those fields, you must pass the names of the fields when setting up a
:class:`.GenericRelation` to it. For example, if the ``TaggedItem`` model
referred to above used fields named ``content_type_fk`` and
``object_primary_key`` to create its generic foreign key, then a
:class:`.GenericRelation` back to it would need to be defined like so::

    tags = generic.GenericRelation(TaggedItem,
                                   content_type_field='content_type_fk',
                                   object_id_field='object_primary_key')

Of course, if you don't add the reverse relationship, you can do the
same types of lookups manually::

    >>> b = Bookmark.objects.get(url='http://www.djangoproject.com/')
    >>> bookmark_type = ContentType.objects.get_for_model(b)
    >>> TaggedItem.objects.filter(content_type__pk=bookmark_type.id,
    ...                           object_id=b.id)
    [<TaggedItem: django>, <TaggedItem: python>]

Note that if the model in a
:class:`~django.contrib.contenttypes.generic.GenericRelation` uses a
non-default value for ``ct_field`` or ``fk_field`` in its
:class:`~django.contrib.contenttypes.generic.GenericForeignKey` (e.g. the
:mod:`django.contrib.comments` app uses ``ct_field="object_pk"``),
you'll need to set ``content_type_field`` and/or ``object_id_field`` in
the :class:`~django.contrib.contenttypes.generic.GenericRelation` to
match the ``ct_field`` and ``fk_field``, respectively, in the
:class:`~django.contrib.contenttypes.generic.GenericForeignKey`::

    comments = generic.GenericRelation(Comment, object_id_field="object_pk")

Note also, that if you delete an object that has a
:class:`~django.contrib.contenttypes.generic.GenericRelation`, any objects
which have a :class:`~django.contrib.contenttypes.generic.GenericForeignKey`
pointing at it will be deleted as well. In the example above, this means that
if a ``Bookmark`` object were deleted, any ``TaggedItem`` objects pointing at
it would be deleted at the same time.

.. versionadded:: 1.3

Unlike :class:`~django.db.models.ForeignKey`,
:class:`~django.contrib.contenttypes.generic.GenericForeignKey` does not accept
an :attr:`~django.db.models.ForeignKey.on_delete` argument to customize this
behavior; if desired, you can avoid the cascade-deletion simply by not using
:class:`~django.contrib.contenttypes.generic.GenericRelation`, and alternate
behavior can be provided via the :data:`~django.db.models.signals.pre_delete`
signal.

Generic relations and aggregation
---------------------------------

:doc:`Django's database aggregation API </topics/db/aggregation>`
doesn't work with a
:class:`~django.contrib.contenttypes.generic.GenericRelation`. For example, you
might be tempted to try something like::

    Bookmark.objects.aggregate(Count('tags'))

This will not work correctly, however. The generic relation adds extra filters
to the queryset to ensure the correct content type, but the
:meth:`~django.db.models.query.QuerySet.aggregate` method doesn't take them
into account. For now, if you need aggregates on generic relations, you'll
need to calculate them without using the aggregation API.

Generic relations in forms and admin
------------------------------------

The :mod:`django.contrib.contenttypes.generic` module provides
:class:`~django.contrib.contenttypes.generic.BaseGenericInlineFormSet`,
:class:`~django.contrib.contenttypes.generic.GenericTabularInline`
and :class:`~django.contrib.contenttypes.generic.GenericStackedInline`
(the last two are subclasses of
:class:`~django.contrib.contenttypes.generic.GenericInlineModelAdmin`).
This enables the use of generic relations in forms and the admin. See the
:doc:`model formset </topics/forms/modelforms>` and
:ref:`admin <using-generic-relations-as-an-inline>` documentation for more
information.

.. class:: GenericInlineModelAdmin

    The :class:`~django.contrib.contenttypes.generic.GenericInlineModelAdmin`
    class inherits all properties from an
    :class:`~django.contrib.admin.InlineModelAdmin` class. However,
    it adds a couple of its own for working with the generic relation:

    .. attribute:: ct_field

        The name of the
        :class:`~django.contrib.contenttypes.models.ContentType` foreign key
        field on the model. Defaults to ``content_type``.

    .. attribute:: ct_fk_field

        The name of the integer field that represents the ID of the related
        object. Defaults to ``object_id``.