search on multiple table

Hello,

I am trying to make a search, using acts_as_ferret on a
has_and_belongs_to_many relation :

class a < ActiveRecord::Base
  belongs_to :b
  has_and_belongs_to_many :c

Joboffer.find_by_contents( query +b_id:1 )     is working fine

but

Joboffer.find_by_contents( query +c:1 )     is not working


Here are the tables descriptions:

Table a have field : id, b_id
Table b have field : id, name
Table c have field : id, name
Table c_a have field :a_id, c_id

Any idea on how to include c on my search ?

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