join_scope

# File activerecord/lib/active_record/reflection.rb, line 190
      def join_scope(table, foreign_klass)
        predicate_builder = predicate_builder(table)
        scope_chain_items = join_scopes(table, predicate_builder)
        klass_scope       = klass_join_scope(table, predicate_builder)

        if type
          klass_scope.where!(type => foreign_klass.polymorphic_name)
        end

        scope_chain_items.inject(klass_scope, &:merge!)
      end