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259 lines
10 KiB
Plaintext
259 lines
10 KiB
Plaintext
==========================
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The contenttypes framework
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==========================
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Django includes a "contenttypes" application that can track all of
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the models installed in your Django-powered project, providing a
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high-level, generic interface for working with your models.
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Overview
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========
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At the heart of the contenttypes application is the ``ContentType``
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model, which lives at
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``django.contrib.contenttypes.models.ContentType``. Instances of
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``ContentType`` represent and store information about the models
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installed in your project, and new instances of ``ContentType`` are
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automatically created whenever new models are installed.
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Instances of ``ContentType`` have methods for returning the model
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classes they represent and for querying objects from those models.
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``ContentType`` also has a `custom manager`_ that adds methods for
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working with ``ContentType`` and for obtaining instances of
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``ContentType`` for a particular model.
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Relations between your models and ``ContentType`` can also be used to
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enable "generic" relationships between an instance of one of your
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models and instances of any model you have installed.
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.. _custom manager: ../model-api/#custom-managers
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Installing the contenttypes framework
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=====================================
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The contenttypes framework is included in the default
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``INSTALLED_APPS`` list created by ``django-admin.py startproject``,
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but if you've removed it or if you manually set up your
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``INSTALLED_APPS`` list, you can enable it by adding
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``'django.contrib.contenttypes'`` to your ``INSTALLED_APPS`` setting.
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It's generally a good idea to have the contenttypes framework
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installed; several of Django's other bundled applications require it:
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* The admin application uses it to log the history of each object
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added or changed through the admin interface.
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* Django's `authentication framework`_ uses it to tie user permissions
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to specific models.
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* Django's comments system (``django.contrib.comments``) uses it to
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"attach" comments to any installed model.
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.. _authentication framework: ../authentication/
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The ``ContentType`` model
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=========================
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Each instance of ``ContentType`` has three fields which, taken
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together, uniquely describe an installed model:
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``app_label``
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The name of the application the model is part of. This is taken from
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the ``app_label`` attribute of the model, and includes only the *last*
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part of the application's Python import path;
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"django.contrib.contenttypes", for example, becomes an ``app_label``
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of "contenttypes".
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``model``
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The name of the model class.
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``name``
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The human-readable name of the model. This is taken from
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`the verbose_name attribute`_ of the model.
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Let's look at an example to see how this works. If you already have
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the contenttypes application installed, and then add `the sites
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application`_ to your ``INSTALLED_APPS`` setting and run ``manage.py
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syncdb`` to install it, the model ``django.contrib.sites.models.Site``
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will be installed into your database. Along with it a new instance
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of ``ContentType`` will be created with the following values:
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* ``app_label`` will be set to ``'sites'`` (the last part of the Python
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path "django.contrib.sites").
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* ``model`` will be set to ``'site'``.
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* ``name`` will be set to ``'site'``.
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.. _the verbose_name attribute: ../model-api/#verbose_name
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.. _the sites application: ../sites/
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Methods on ``ContentType`` instances
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====================================
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Each ``ContentType`` instance has methods that allow you to get from a
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``ContentType`` instance to the model it represents, or to retrieve objects
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from that model:
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``get_object_for_this_type(**kwargs)``
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Takes a set of valid `lookup arguments`_ for the model the
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``ContentType`` represents, and does `a get() lookup`_ on that
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model, returning the corresponding object.
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``model_class()``
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Returns the model class represented by this ``ContentType``
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instance.
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For example, we could look up the ``ContentType`` for the ``User`` model::
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>>> from django.contrib.contenttypes.models import ContentType
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>>> user_type = ContentType.objects.get(app_label="auth", model="user")
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>>> user_type
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<ContentType: user>
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And then use it to query for a particular ``User``, or to get access
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to the ``User`` model class::
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>>> user_type.model_class()
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<class 'django.contrib.auth.models.User'>
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>>> user_type.get_object_for_this_type(username='Guido')
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<User: Guido>
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Together, ``get_object_for_this_type`` and ``model_class`` enable two
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extremely important use cases:
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1. Using these methods, you can write high-level generic code that
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performs queries on any installed model -- instead of importing and
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using a single specific model class, you can pass an ``app_label``
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and ``model`` into a ``ContentType`` lookup at runtime, and then
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work with the model class or retrieve objects from it.
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2. You can relate another model to ``ContentType`` as a way of tying
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instances of it to particular model classes, and use these methods
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to get access to those model classes.
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Several of Django's bundled applications make use of the latter
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technique. For example, `the permissions system`_ in Django's
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authentication framework uses a ``Permission`` model with a foreign
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key to ``ContentType``; this lets ``Permission`` represent concepts
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like "can add blog entry" or "can delete news story".
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.. _lookup arguments: ../db-api/#field-lookups
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.. _a get() lookup: ../db-api/#get-kwargs
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.. _the permissions system: ../authentication/#permissions
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The ``ContentTypeManager``
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--------------------------
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``ContentType`` also has a custom manager, ``ContentTypeManager``,
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which adds the following methods:
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``clear_cache()``
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Clears an internal cache used by ``ContentType`` to keep track of which
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models for which it has created ``ContentType`` instances. You probably
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won't ever need to call this method yourself; Django will call it
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automatically when it's needed.
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``get_for_model(model)``
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Takes either a model class or an instance of a model, and returns the
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``ContentType`` instance representing that model.
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The ``get_for_model`` method is especially useful when you know you
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need to work with a ``ContentType`` but don't want to go to the
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trouble of obtaining the model's metadata to perform a manual lookup::
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>>> from django.contrib.auth.models import User
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>>> user_type = ContentType.objects.get_for_model(User)
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>>> user_type
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<ContentType: user>
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Generic relations
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=================
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Adding a foreign key from one of your own models to ``ContentType``
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allows your model to effectively tie itself to another model class, as
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in the example of the ``Permission`` model above. But it's possible to
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go one step further and use ``ContentType`` to enable truly generic
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(sometimes called "polymorphic") relationships between models.
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A simple example is a tagging system, which might look like this::
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from django.db import models
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from django.contrib.contenttypes.models import ContentType
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from django.contrib.contenttypes import generic
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class TaggedItem(models.Model):
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tag = models.SlugField()
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content_type = models.ForeignKey(ContentType)
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object_id = models.PositiveIntegerField()
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content_object = generic.GenericForeignKey('content_type', 'object_id')
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def __unicode__(self):
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return self.tag
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A normal ``ForeignKey`` can only "point to" one other model, which
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means that if the ``TaggedItem`` model used a ``ForeignKey`` it would have to
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choose one and only one model to store tags for. The contenttypes
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application provides a special field type --
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``django.contrib.contenttypes.generic.GenericForeignKey`` -- which
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works around this and allows the relationship to be with any
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model. There are three parts to setting up a ``GenericForeignKey``:
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1. Give your model a ``ForeignKey`` to ``ContentType``.
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2. Give your model a field that can store a primary-key value from the
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models you'll be relating to. (For most models, this means an
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``IntegerField`` or ``PositiveIntegerField``.)
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3. Give your model a ``GenericForeignKey``, and pass it the names of
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the two fields described above. If these fields are named
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"content_type" and "object_id", you can omit this -- those are the
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default field names ``GenericForeignKey`` will look for.
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This will enable an API similar to the one used for a normal ``ForeignKey``;
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each ``TaggedItem`` will have a ``content_object`` field that returns the
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object it's related to, and you can also assign to that field or use it when
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creating a ``TaggedItem``::
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>>> from django.contrib.auth.models import User
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>>> guido = User.objects.get(username='Guido')
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>>> t = TaggedItem(content_object=guido, tag='bdfl')
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>>> t.save()
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>>> t.content_object
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<User: Guido>
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Reverse generic relations
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-------------------------
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If you know which models you'll be using most often, you can also add
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a "reverse" generic relationship to enable an additional API. For example::
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class Bookmark(models.Model):
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url = models.URLField()
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tags = generic.GenericRelation(TaggedItem)
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``Bookmark`` instances will each have a ``tags`` attribute, which can
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be used to retrieve their associated ``TaggedItems``::
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>>> b = Bookmark(url='http://www.djangoproject.com/')
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>>> b.save()
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>>> t1 = TaggedItem(content_object=b, tag='django')
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>>> t1.save()
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>>> t2 = TaggedItem(content_object=b, tag='python')
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>>> t2.save()
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>>> b.tags.all()
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[<TaggedItem: django>, <TaggedItem: python>]
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If you don't add the reverse relationship, you can do the lookup manually::
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>>> b = Bookmark.objects.get(url='http://www.djangoproject.com/)
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>>> bookmark_type = ContentType.objects.get_for_model(b)
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>>> TaggedItem.objects.filter(content_type__pk=bookmark_type.id,
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... object_id=b.id)
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[<TaggedItem: django>, <TaggedItem: python>]
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Note that if you delete an object that has a ``GenericRelation``, any objects
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which have a ``GenericForeignKey`` pointing at it will be deleted as well. In
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the example above, this means that if a ``Bookmark`` object were deleted, any
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``TaggedItem`` objects pointing at it would be deleted at the same time.
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