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.
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.
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:
Each instance of ContentType has three fields which, taken together, uniquely describe an installed 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:
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:
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".
ContentType also has a custom manager, ContentTypeManager, which adds the following methods:
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>
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:
Give your model a ForeignKey to ContentType.
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.
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)
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.
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.
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.
The GenericInlineModelAdmin class inherits all properties from an InlineModelAdmin class. However, it adds a couple of its own for working with the generic relation:
Jul 05, 2010