How to Model a Foreign Key in a Reusable Django App?

Question

In my django site I have two apps, blog and links. blog has a model blogpost, and links has a model link. There should be a one to many relationship between these two things. There are many links per blogpost, but each link has one and only one blog post. The simple answer is to put a ForeignKey to blogpost in the link model.

That's all well and good, however there is a problem. I want to make the links app reusable. I don't want it to depend upon the blog app. I want to be able to use it again in other sites and perhaps associate links with other non-blogpost apps and models.

A generic foreign key seems like it might be the answer, but not really. I don't want links to be able to associate with any model in my site. Just the one that I explicitly specify. And I know from prior experience that there can be issues using generic foreign keys in terms of database usage because you can't do a select_related over a generic foreign key the way you can with a regular foreign key.

What is the "correct" way to model this relationship?

Answer 1

If you think the link app will always point to a single app then one approach would be to pass the name of the foreign model as a string containing the application label instead of a class reference (Django docs explanation).

In other words, instead of:

class Link(models.Model):
    blog_post = models.ForeignKey(BlogPost)

do:

from django.conf import setings
class Link(models.Model):
    link_model = models.ForeignKey(settings.LINK_MODEL)

and in your settings.py:

LINK_MODEL = 'someproject.somemodel'

Answer 2

I think TokenMacGuy is on the right track. I would look at how django-tagging handles a similar generic relationship using the content type, generic object_id, and generic.py. From models.py

class TaggedItem(models.Model):
    """
    Holds the relationship between a tag and the item being tagged.
    """
    tag          = models.ForeignKey(Tag, verbose_name=_('tag'), related_name='items')
    content_type = models.ForeignKey(ContentType, verbose_name=_('content type'))
    object_id    = models.PositiveIntegerField(_('object id'), db_index=True)
    object       = generic.GenericForeignKey('content_type', 'object_id')

    objects = TaggedItemManager()

    class Meta:
        # Enforce unique tag association per object
        unique_together = (('tag', 'content_type', 'object_id'),)
        verbose_name = _('tagged item')
        verbose_name_plural = _('tagged items')