through_scope_attributes

# File activerecord/lib/active_record/associations/has_many_through_association.rb, line 70
        def through_scope_attributes
          scope = through_scope || self.scope
          scope.where_values_hash(through_association.reflection.name.to_s).
            except!(through_association.reflection.foreign_key,
                    through_association.reflection.klass.inheritance_column)
        end