Active scaffold with attibute in join table

hi,
I've a many to many relationship:

class Operation < ActiveRecord::Base
  has_and_belongs_to_many :roles
end

class Role < ActiveRecord::Base
  has_and_belongs_to_many :operations
end

Roles table contains the users roles and operations table the operations
of the application.

I'm using active scaffold and i've created the operations_rules join
table with those attribute:

operation_id, rule_id, create (Permits to create)

how can I tell Active scaffold to add the create attibute in the subform
(on the operation controller)? and how save this attribute on the join
table?

Thanks.