map.resource :has_many :through :has_and_belongs_to_many

Hello,
I've got the following model

objectA 1 <------> 0..* objectB 0..* <-------> 1 objectC

I'd like the url
GET /objectA/1/objectC
to show all objectC associated with objectA through objectB

In routes.rb
map.resources :objectAs, :has_many => [:objectCs]
or
map.resources :objectAs do |:objectA|
objectA.resources :objectCs
end

shows me all objectCs not just those associated with objectA through
objectB

How can I get all objectCs associated with objectA using the url GET /
objectA/1/objectC?
Also if I had the following model
objectA 0..* <-------> 0..* objectC
How can I get all objectCs associated with objectA using the url GET /
objectA/1/objectC?

Thanks,
ewan