Let GenericForeignKeys behave like a ForeignKey w.r.t. unsaved objects

[Willem Van Onsem]

As mentioned on ​StackOverflow, a ForeignKey can still save an object if the object was not saved when setting the model object, like:

one = Model_One()
two = Model_Two(model_one=one)
one.save()
two.save()

But this is not the case for a GenericForeignKey with:

one = Model_One()
two = Model_With_GFK(content_object=one)
one.save()
two.save()

This is because the content_object *immediately* sets the content type field and the foreign key field when setting the object, and then leaves is that way.

Perhaps overkill, but it might be worth to do the same for a ForeignKey and thus check if the object has been given .pk, and if so, set the foreign key field.