Add support for before and after add/remove callbacks to belongs_to associations

belongs_to currently does not support before/after_add/remove callbacks as has_many does. There are a variety of applications in which this is beneficial. I found an original ticket from a few years ago but it only implemented the before/after_add callbacks and did not apply cleanly any more.

I've reworked the existing patch from the link below to support all necessary callbacks including before_add, before_remove, after_add, and after_remove. More information on the original request can be found on the ticket here:

http://dev.rubyonrails.org/ticke...

Thanks!

• sob