Filter Next/Previous by Relation

[George Hickman]

next() & previous() methods on Orderable filter all orderable models so update them to take an optional relation and filter on.

Example

[smcoll]

+1. It would be great to have models orderable by relation. That's my primary use case.

[Marc Tamlyn]

These methods now use get_filtered_manager() so they will respect Meta.unique_together. I think this can be closed.

[Charlie Denton]

I think that @mjtamlyn's patch does indeed fix this, @ghickman -- the example link is broken however, so please let me know if this should be reopened.