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Deferred foreign keys for Django

Project description


django-dfk implements deferred foreign keys for Django. Deferred foreign keys are conceptually
similar to generic foreign keys, except that they are resolved to a real foreign key at runtime,
and cause proper foreign keys to be created in the database.

This package allows you to do two things::

* Declare that a model's foreign key field is 'deferrable', and should be repointed later
* Repoint an existing model's foreign key fields, even if that model is not django-dfk aware.

You should perform the latter with caution - consider it a similar process to monkey-patching!

This package is alpha software, and is not feature-complete. See the TODO section for what's
on the list.

django-dfk is compatible with Python 2.6, 2.7, 3.2 and 3.3.
django-dfk is compatible with django versions 1.3 - 1.7


Install ``django-dfk`` using your preferred Python package manager. Use of ``virtualenv`` is
also recommended::

pip install django-dfk


Pointing a single foreign key

Let's say you want to reinvent the wheel, and develop a commenting app. Your comment model
might look like this, in ``mycomments.models``::

from dfk import DeferredForeignKey

class Comment(models.Model):
commenter = models.ForeignKey('auth.User')
content = DeferredForeignKey()
body = models.TextField()

Now, you come to integrate this application with your blog system (which, as you're keen
on wheel reinvention, you have also written yourself). Here's ``blog/``::

from dfk import point
from mycomments.models import Comment

class BlogPost(models.Model):
title = models.CharField(max_length=100)
slug = models.SlugField()
body = models.TextField()

point(Comment, 'content', BlogPost)

The call to ``point`` will replace the ``DeferredForeignKey`` on ``Comment`` with a foreign key to BlogPost.

Pointing many foreign keys at once

When writing models that use deferred foreign keys, you may need to declare that a number
should point to the same 'kind' of object. Let's say you had wild scope creep, and your
commenting app needed the ability to associate images with a blog post. So you edit
your comment app's so it looks like this::

from dfk import DeferredForeignKey

class Comment(models.Model):
commenter = models.ForeignKey('auth.User')
content = DeferredForeignKey(name='Content')
body = models.TextField()

class Image(models.Model):
image = models.ImageField()
content = DeferredForeignKey(name='Content')

This expresses that both comments and images need to point to the same kind of model. This is
accomplished with the ``point_named`` function::

from dfk import point_named
point_named('blog', 'Content', BlogPost)

Now, all ``DeferredForeignKey`` instances in the ``blog`` app which are called ``Content`` will
be replaced by real foreign keys to ``BlogPost``.

Arguments to the generated foreign keys

When declaring a deferred foreign key, you may specify additional keyword arguments. Aside from
``name``, this will be passed on verbatim to the final foreign key.

It is also possible to pass arbitrary keyword arguments in calls to ``point`` or ``point_named``.
These will also be passed to the final foreign key. Where arguments are present in both the
DFK definition and in the ``point``/``point_named`` call, arguments from the latter will take

Model inheritance

Model inheritance should Just Work. It's possible to have ``DeferredForeignKey``
instances on subclasses and base classes. The only thing to be aware of is that
repointing a dfk on a subclass where the key is actually defined on a
non-abstract base class is illegal, and will raise a ``TypeError``.

Cleaning object caches

Pointing or repointing foreign keys requires that related object caches are
repopulated as relationships will have changed and things like filtering on
related objects are likely to fail.

By default object caches are cleaned after each ``point`` or ``repoint``.
For apps with many ``DeferredForeignKey`` instances involving the same model
it may be more efficient to clean the caches once, after all pointing and
repointing has finished. To enable this pass ``clean_caches=False`` to
``point`` or ``repoint`` and then manually call ``clean_object_caches`` as

from dfk import point
from dfk import clean_object_caches
from mycomments.models import Comment

class BlogPost(models.Model):
title = models.CharField(max_length=100)
slug = models.SlugField()
body = models.TextField()

point(Comment, 'content', BlogPost, clean_caches=False)
clean_object_caches(Comment, BlogPost)


Thanks to ISM Fantasy Games Ltd. for sponsoring this package.
Package maintained by Dan Fairs, Rob Charlwood and Ian Dash

- Setup Tox environments for Django 1.7
- Added MIDDLEWARE_CLASSES definition to test settings to avoid warnings from
Django 1.7's system check.
- Added install_requires limits to to reflect supported versions of
- Updated docs.

- Setup Tox environments for Django 1.6
- Fixed repoint issues under Django 1.6 due to use of new ForeignObject baseclass for related objects.

- Make codebase Python 3 compatible (3.2, 3.3) (
- Make codebase compatible with django 1.5.4 (
- New Python 3 compatible codebase has been tested against django 1.5.4 (
- Carried out full tidy up with PEP8 compliance. (
- Setup and configured ``django-dfk`` for use with ``tox`` testing library (
- Added full documentation on running test suite (


Make cache cleaning optional, and fix some PEP8 compliance issues. Thanks
to Mark Hughes (


Fix a problem where repointing a deferred foreign key defined on a non-abstract
base class through a subclass would result in a new field being added to the
local_fields of the subclass, shadowing the one on the base class. It is now
illegal to do this; dfks on base classes should be pointed using the base class


Fix a problem where related object caches on models' _meta Options classes
were not being repopulated on a repoint. This led to problems where
filtering on a parent model related to a child using a deferred foreign key
could fail if the dfk was (re)pointed after the initial phase of model loading
had already taken place.


- Include a to ensure docs are packaged.

- Fix packaging error


- Fix an issue when repointing foreign keys on model classes with custom
fields which use the django.db.models.SubfieldBase metaclass
- Fix an issue migrating from Django 1.2 to 1.3.


- Initial version

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