activerecord: how can I get polymorphic entities with different collections loaded in 1 go?

Question

ok, so i got

class A < ActiveRecord::Base
  has_and_belongs_to_many :cs
and 

class Aa < A
end

class Ab < A
  belongs_to :b
end

How can I get all As(the base class) of a certain c, with the Abs joined to their bs - in one go? Currently, I load a certain c, then go c.as, but I cannot manage to get their bs, too...