# File lib/mongoid/contexts/mongo.rb, line 362
      def grouped(start, field, reduce)
        collection = klass.collection.group(
          :cond => selector,
          :initial => { start => "start" },
          :reduce => reduce.gsub("[field]", field)
        )
        value = collection.empty? ? nil : collection.first[start.to_s]
        value && value.do_or_do_not(:nan?) ? nil : value
      end