search for: unprocessed_b

I have two models, A and B, with a straightforward many-to-many has_many/through association between them. So, in A I have the association processed_bs and in B I have processedby_as. This works fine. Now, I would like to also have an association unprocessed_bs in A, which holds all Bs that _aren''t_ in processed_bs. I can write a method in A that returns all those objects as a list (using B.find(:all,:conditions)), but that''s an assymetric and not very pleasing solution. For example it means that I can to aobj.processed_bs.count but not...