# File lib/active_record/aggregations.rb, line 144
      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