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Fixed #14820 -- Added more information to the generic relation docs r…

…egarding different choices for storing PK references for a GenericForeignKey. Thanks to mrmachine for the all the work on the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15545 bcc190cf-cafb-0310-a4f2-bffc1f526a37
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commit 9323f81dc03047371329c954c8eec8ef97d4dc16 1 parent e9d99c4
Gabriel Hurley authored
Showing with 25 additions and 7 deletions.
  1. +25 −7 docs/ref/contrib/contenttypes.txt
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32 docs/ref/contrib/contenttypes.txt
@@ -251,13 +251,8 @@ model:
2. 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
- :class:`~django.db.models.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.
+ :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
@@ -267,6 +262,29 @@ model:
: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.ForeignKey`;
each ``TaggedItem`` will have a ``content_object`` field that returns the
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