intersection of join models?

We have models A, B, C such that

   A has_many B :through C

Given a in A, b in B, is there a direct way to ask for the cs in C that belong to both without using explicit IDs in AR calls? I got

   a.cs & b.cs

but involves two queries.

-- fxn

Because AR is designed in such a way that using an _id column explicitely for simple things triggers a warning in my head, since it usually means I am not using the abstractions correctly.

We can here use #detect on a.cs, or a dynamic finder a.cs.find_by_b_id(, but looks non-idiomatic, so I asked whether I am missing something. Perhaps in this particular case there is no prepackaged way to do this, in which case I'll remove the auto-warning :-).

-- fxn