    # File lib/active_record/aggregations.rb, line 135
          def composed_of(part_id, options = {}, &block)
            options.assert_valid_keys(:class_name, :mapping, :allow_nil)

            name        = part_id.id2name
            class_name  = options[:class_name] || name.camelize
            mapping     = options[:mapping]    || [ name, name ]
            mapping     = [ mapping ] unless mapping.first.is_a?(Array)
            allow_nil   = options[:allow_nil]  || false

            reader_method(name, class_name, mapping, allow_nil)
            writer_method(name, class_name, mapping, allow_nil, block)

            create_reflection(:composed_of, part_id, options, self)
          end