code: public <S, L> MapReduceOutput<S, L> mapReduce(MapReduce.MapReduceCommand<S, L> command) throws MongoException {
          return new MapReduceOutput<S, L>(this, dbCollection.mapReduce(command.build(this)), command.getResultType(),
                  command.getKeyType());
      }
docs: Performs a map reduce operation
  
  @param command The command to execute
  @return The output
  @throws MongoException If an error occurred