# File lib/active_record/aggregations.rb, line 122
      def composed_of(part_id, options = {})
        options.assert_valid_keys(:class_name, :mapping)

        name        = part_id.id2name
        class_name  = options[:class_name] || name_to_class_name(name)
        mapping     = options[:mapping] || [ name, name ]

        reader_method(name, class_name, mapping)
        writer_method(name, class_name, mapping)
      end