How it works...

As you can see, this snippet is more complex than the previous ones.

The object_relation_base_factory function, which we have aliased to generic_relation, for short, in our import, is not a mixin itself; it is a function that generates a model mixin – that is, an abstract model class to extend from. The dynamically created mixin adds the content_type and object_id fields and the content_object generic foreign key that points to the related instance.

Why can't we just define a simple model mixin with these three attributes? A dynamically generated abstract class allows us to have prefixes for each field name; therefore, we can have more than one generic relation in the same model. For example, the Like model, which was shown previously, will have the content_type, object_id, and content_object fields for the favorite object, and owner_content_type, owner_object_id, and owner_content_object for the one (user or group) that liked the object.

The object_relation_base_factory function, which we have aliased
to generic_relation for short, adds the possibility to limit the content type choices by the limit_content_type_choices_to parameter. The preceding example limits the choices for owner_content_type to only the content types of the User and Group models.