public function orderBy($field, $order = 'ASC')
    {
        if(is_string($field) && trim($field) != '') {
            $field = trim($field);
            $field = ltrim($field, 'ORDER BY ');
            $this->order = ' ORDER BY `'.$field.'`'.(($order == 'DESC') ? ' DESC' : ' ASC');
        }
        return $this;
    }