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
django-dfk using your preferred Python package manager. Use of
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
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
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
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 models.py 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
from dfk import point_named point_named('blog', 'Content', BlogPost)
DeferredForeignKey instances in the
blog app which are called
be replaced by real foreign keys to
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
These will also be passed to the final foreign key. Where arguments are present in both the
DFK definition and in the
point_named call, arguments from the latter will take
Model inheritance should Just Work. It's possible to have
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
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
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
repoint and then manually call
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