protected function canDelete($field_value_id)
    {
        $sql = 'SELECT c.product_id
                FROM product_field pf
                LEFT JOIN cart c ON(pf.product_id = c.product_id)
                WHERE pf.field_value_id=?';

        $result = $this->db->fetchColumn($sql, array($field_value_id));
        return empty($result);
    }