protected function _addKey($key)
    {
        if ($key == '__stored_keys__') {
            return $this;
        }

        $keys = $this->get('__stored_keys__', []);
        if (!in_array($key, $keys)) {
            array_push($keys, $key);
        }
        $this->set('__stored_keys__', $keys, 24*60*60);
        return $this;
    }