Rails - May 2006 - Complement set association to has_many/through?

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 aobj.unprocessed_bs.count. Which is
clearly ugly.

I''ve tried playing around with feeding various variants of :select and
:finder_sql to the has_many declaration, but so far only produced lots
of different tracebacks. I''ve also got this ugly feeling that writing
that much SQL is an invitation to problems when/if I want to switch
databases. 

So is there a neat way to get the complement set to an existing
association?

-- 
		     Calle Dybedahl <calle@cyberpomo.com>
		 http://www.livejournal.com/users/cdybedahl/
	  "Data on the network has cooties and will EAT YOUR BRAINS"
			   -- Ross Younger, BofhNet