public function orderDesc($field, $overwrite = false)
    {
        if ($overwrite) {
            $this->_parts['order'] = null;
        }
        if (!$field) {
            return $this;
        }

        if (!$this->_parts['order']) {
            $this->_parts['order'] = new OrderByExpression();
        }
        $this->_parts['order']->add(new OrderClauseExpression($field, 'DESC'));

        return $this;
    }