public function orderBy($field, $mode = 'ASC')
    {
        $mode = strtoupper($mode);
        if ( ! in_array($mode, ['ASC', 'DESC'])) {
            throw new \InvalidArgumentException("Error order by mode");
        }
        // is the first time call orderBy method ?
        if ($this->_orderby_str == '') {
            $this->_orderby_str = ' ORDER BY '.self::_wrapRow($field).' '.$mode;
        } else {
            $this->_orderby_str .= ' , '.self::_wrapRow($field).' '.$mode;
        }

        return $this;
    }