order_with_respect_to GenericForeignKey

[Alex Hill]

As of 1.8, order_with_respect_to nearly works with generic relations.

First thing that's broken is the get_RELATED_order and set_RELATED_order methods. They are set in ​ModelBase._prepare. You don't know what model is going to be on the other end of the GenericForeignKey, so I think those can simply be omitted if opts.order_with_respect_to.rel is None. (Perhaps something can be done here in cases where a GenericRelation is defined.)

Next, you run into trouble ​when Django tries to save a new object's _order field, because there's no attname attribute on the GenericForeignKey. You run into the same problem in _get_next_or_previous_in_order().

This is easily solved if you're OK with sprinkling details of the GenericForeignKey implementation in Django core: you can just try attname, and if that fails, filter on GFK's ct_field and pk_field. But maybe a better idea would be to introduce a method on field-like things returning a dictionary of filter parameters matching a given object, or else a property returning a sequence of (filter name, attribute name) pairs from which the filter parameters can be constructed. In the latter case, regular fields would return ((self.name, self.attname),), and GFKs would return ((self.fk_field, self.fk_field), (self.ct_field, self.ct_field)).