code: public static function groupBy($field)
      {
          if (!(gettype($field)=="string" || $field instanceof Key || $field instanceof Raw)) {
              throw new ClusterpointException("\"->group()\" function: passed field selector is not in valid format.", 9002);
          }
          if ($field instanceof Key) {
              $field =  self::field("{$field}");
          }
          return "{$field}";
      }
docs: Pass back GROUP BY Clause to append the scope.
  
  @param  mixed  $field
  @return string