# File lib/mongoid/criterion/exclusion.rb, line 71
      def only(*args)
        clone.tap do |crit|
          if args.any?
            crit.options[:fields] = {:_type => 1}
            crit.field_list = args.flatten
            crit.field_list.each do |f|
              crit.options[:fields][f] = 1
            end
          end
        end
      end