code: public function populate($rows)
      {
          if ($this->indexBy === null) {
              return $rows;
          }
          $result = [];
          foreach ($rows as $row) {
              $result[ArrayHelper::getValue($row, $this->indexBy)] = $row;
          }
  
          return $result;
      }
docs: Converts the raw query results into the format as specified by this query.
  This method is internally used to convert the data fetched from database
  into the format as required by this query.
  @param array $rows the raw query result from database
  @return array the converted query result