我应该在类析构函数中保存数据吗?

这是我的类，这个类的实例知道什么时候应该保存值。你怎么看这个想法?这个实现的优点和缺点是什么?

class Model_User_Doc extends ArrayObject {
    protected $_id;
    protected $_value;
    protected $_db;
    protected $_changed = FALSE;
    public function __construct($id = NULL) {
        if ($id !== NULL) {
            $this->_id = $id;
            $this->_db = DB::instance();
            $this->_value = $this->_db->get($id);
        }
    }
    public function __set($key, $value) {
        $this->_changed = TRUE;
        $this->_value[$key] = $value;
    }
    public function __get($key) {
        if (isset($this->_value[$key])) {
            return $this->_value[$key];
        }
        return NULL;
    }
    public function __unset($key) {
        $result = FALSE;        
        if (isset($this->_value[$key])) {
            $this->_changed = TRUE;
            unset($this->_value[$key]);
            $result = TRUE;
        }
        return $result;
    }
    public function offsetGet($name) {
        return $this->_value[$name];
    }
    public function offsetSet($name, $value) {
        $this->_changed = TRUE;
        $this->_value[$name] = $value;
    }
    public function offsetExists($name) {
        return isset($this->_value[$name]);
    }
    public function offsetUnset($name) {
        $this->_changed = TRUE;
        unset($this->_value[$name]);
    }
    public function cas() {
        if ($this->_changed === TRUE) {
            $this->save();
        }
    }
    public function save() {
        $this->_db->set($this->_id, $this->_value);
        $this->_changed = FALSE;
    }
    public function __destruct() {
        $this->cas();
    }
}
?>