Django documentation

This document is for Django's SVN release, which can be significantly different from previous releases. Get old docs here: Django 1.0

The contenttypes framework

Django includes a 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 ContentType model, which lives at django.contrib.contenttypes.models.ContentType. Instances of ContentType represent and store information about the models installed in your project, and new instances of ContentType are automatically created whenever new models are installed.

Instances of ContentType have methods for returning the model classes they represent and for querying objects from those models. ContentType also has a custom manager that adds methods for working with ContentType and for obtaining instances of ContentType for a particular model.

Relations between your models and 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 INSTALLED_APPS list created by django-admin.py startproject, but if you’ve removed it or if you manually set up your INSTALLED_APPS list, you can enable it by adding 'django.contrib.contenttypes' to your 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 authentication framework uses it to tie user permissions to specific models.
  • Django’s comments system (django.contrib.comments) uses it to “attach” comments to any installed model.

The ContentType model

class models.ContentType

Each instance of ContentType has three fields which, taken together, uniquely describe an installed model:

app_label
The name of the application the model is part of. This is taken from the 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 app_label of “contenttypes”.
model
The name of the model class.
name
The human-readable name of the model. This is taken from the verbose_name attribute of the model.

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

  • app_label will be set to 'sites' (the last part of the Python path “django.contrib.sites”).
  • model will be set to 'site'.
  • name will be set to 'site'.

Methods on ContentType instances

class models.ContentType
Each ContentType instance has methods that allow you to get from a ContentType instance to the model it represents, or to retrieve objects from that model:
models.ContentType.get_object_for_this_type(**kwargs)
Takes a set of valid lookup arguments for the model the ContentType represents, and does a get() lookup on that model, returning the corresponding object.
models.ContentType.model_class()
Returns the model class represented by this ContentType instance.

For example, we could look up the ContentType for the 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 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, get_object_for_this_type() and 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 ContentType lookup at runtime, and then work with the model class or retrieve objects from it.
  2. You can relate another model to 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, the permissions system in Django's authentication framework uses a Permission model with a foreign key to ContentType; this lets Permission represent concepts like "can add blog entry" or "can delete news story".

The ContentTypeManager

class models.ContentTypeManager

ContentType also has a custom manager, ContentTypeManager, which adds the following methods:

clear_cache()
Clears an internal cache used by ContentType to keep track of which models for which it has created 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.
get_for_model(model)
Takes either a model class or an instance of a model, and returns the ContentType instance representing that model.

The get_for_model() method is especially useful when you know you need to work with a 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>

Generic relations

Adding a foreign key from one of your own models to ContentType allows your model to effectively tie itself to another model class, as in the example of the Permission model above. But it's possible to go one step further and use 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 ForeignKey can only "point to" one other model, which means that if the TaggedItem model used a ForeignKey it would have to choose one and only one model to store tags for. The contenttypes application provides a special field type -- django.contrib.contenttypes.generic.GenericForeignKey -- which works around this and allows the relationship to be with any model. There are three parts to setting up a GenericForeignKey:

  1. Give your model a ForeignKey to ContentType.

  2. Give your model a field that can store a primary-key value from the models you'll be relating to. (For most models, this means an IntegerField or PositiveIntegerField.)

    This field must be of the same type as the primary key of the models that will be involved in the generic relation. For example, if you use IntegerField, you won't be able to form a generic relation with a model that uses a CharField as a primary key.

  3. Give your model a 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 GenericForeignKey will look for.

This will enable an API similar to the one used for a normal 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 GenericForeignKey is implemented, you cannot use such fields directly with filters (filter() and exclude(), for example) via the database API. They aren't 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

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 django.contrib.contenttypes.generic.GenericForeignKey accepts the names of the content-type and object-ID fields as arguments, so too does 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 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 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 with a GenericForeignKey that you're referring to uses a non-default value for ct_field or fk_field (e.g. the django.contrib.comments app uses ct_field="object_pk"), you'll need to pass content_type_field and object_id_field to GenericRelation.:

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

Note that if you delete an object that has a GenericRelation, any objects which have a 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.

Generic relations and aggregation

Django's database aggregation API doesn't work with a 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 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

django.contrib.contenttypes.generic provides both a GenericInlineFormSet and GenericInlineModelAdmin. This enables the use of generic relations in forms and the admin. See the model formset and admin documentation for more information.

class generic.GenericInlineModelAdmin

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

ct_field
The name of the ContentType foreign key field on the model. Defaults to content_type.
ct_fk_field
The name of the integer field that represents the ID of the related object. Defaults to object_id.

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