protected function generateId()
    {
        static $choices = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
        $id = '';
        while (strlen($id) < 9) {
            $id .= $choices[ mt_rand(0, strlen($choices) - 1) ];
        }
        return $id;
    }