private function _formatOrder()
    {
        $orders = array();

        if (isset($this->_order[DbInterface::ORDER_RAND])) {
            $orders[] = 'RAND()';
        } else {
            foreach ($this->_order as $field => $order) {
                $orders[] = "`$field` $order";
            }
        }

        return ' ORDER BY ' . implode(', ', $orders);
    }