protected function getNextId()
    {
        $keys = $this->getFileManager()->keys();
        $ids = array();
        foreach($keys as $key) {
            if(preg_match('/^'.$this->getHash().'/', $key)) {
                $ids[] = str_replace($this->getHash(), '', $key);
            }
        }
        if(empty($ids)) {
            return 1;
        }
        
        $maxId = max($ids);
        
        return $maxId + 1;
    }