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.
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 ContentType model¶
- class ContentType¶
Each instance of ContentType has three fields which, taken together, uniquely describe an installed model:
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”.
The name of the model class.
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:
Methods on ContentType instances¶
>>> 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>
- 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.
- 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".
- class ContentTypeManager¶
Clears an internal cache used by ContentType to keep track of which models for which it has created ContentType instances. You probably won't ever need to call this method yourself; Django will call it automatically when it's needed.
Takes either a model class or an instance of a model, and returns the ContentType instance representing that model.
>>> 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 which works around this and allows the relationship to be with any model:
- class GenericForeignKey¶
There are three parts to setting up a GenericForeignKey:
- Give your model a ForeignKey to ContentType.
- 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 PositiveIntegerField. The usual name for this field is "object_id".
- 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.
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 get_db_prep_value() method.
For example, if you want to allow generic relations to models with either IntegerField or CharField primary key fields, you can use CharField for the "object_id" field on your model since integers can be coerced to strings by get_db_prep_value().
For maximum flexibility you can use a 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.
Serializing references to ContentType objects
If you're serializing data (for example, when generating fixtures) from a model that implements generic relations, you should probably be using a natural key to uniquely identify related ContentType objects. See natural keys and dumpdata --natural for more information.
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. Because a 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 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 in a GenericRelation uses a non-default value for ct_field or fk_field in its GenericForeignKey (e.g. the django.contrib.comments app uses ct_field="object_pk"), you'll need to set content_type_field and/or object_id_field in the GenericRelation to match the ct_field and fk_field, respectively, in the GenericForeignKey:
comments = generic.GenericRelation(Comment, object_id_field="object_pk")
Note also, 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.
Unlike ForeignKey, GenericForeignKey does not accept an on_delete argument to customize this behavior; if desired, you can avoid the cascade-deletion simply by not using GenericRelation, and alternate behavior can be provided via the pre_delete signal.
Generic relations and aggregation¶
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¶
The django.contrib.contenttypes.generic module provides BaseGenericInlineFormSet, GenericTabularInline and GenericStackedInline (the last two are subclasses of GenericInlineModelAdmin). This enables the use of generic relations in forms and the admin. See the model formset and admin documentation for more information.
- class GenericInlineModelAdmin¶
The name of the integer field that represents the ID of the related object. Defaults to object_id.