private function setDirectionByString($direction)
    {
        $direction = mb_strtolower($direction);

        switch ($direction) {
            case 'ascending':
            case 'asc':
                $this->direction = self::ORDER_BY_DIR_ASCENDING;
                break;

            case 'descending':
            case 'desc':
                $this->direction = self::ORDER_BY_DIR_DESCENDING;
                break;

            default:
                $this->direction = self::ORDER_BY_DIR_NONE;
        }
    }