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[1.2.X] Fixed #14820 -- Added more information to the generic relation docs regarding different choices for storing PK references for a GenericForeignKey. Thanks to mrmachine for the all the work on the patch.

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Backport of [15545] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15546 bcc190cf-cafb-0310-a4f2-bffc1f526a37
This commit is contained in:
Gabriel Hurley 2011-02-16 00:21:27 +00:00
parent ddc9cefc21
commit 95a6d5a2ef
1 changed files with 26 additions and 9 deletions
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35
docs/ref/contrib/contenttypes.txt
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@ -238,15 +238,9 @@ model. There are three parts to setting up a
to :class:`~django.contrib.contenttypes.models.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
:class:`~django.db.models.fields.IntegerField` or
:class:`~django.db.models.fields.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
:class:`~django.db.models.fields.IntegerField`, you won't be able to
form a generic relation with a model that uses a
:class:`~django.db.models.fields.CharField` as a primary key.
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
@ -256,6 +250,29 @@ model. There are three parts to setting up a
: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.
This will enable an API similar to the one used for a normal
:class:`~django.db.models.fields.related.ForeignKey`;
each ``TaggedItem`` will have a ``content_object`` field that returns the