我应该使用观察者模式吗？

User
  id
  name
  email
  ...

Order
  id
  user_id
  total
  ...

class User extends ORM { }

$user = new User(1);
$user->name = \'New Name\'
$user->save();

class User extends ORM implements SplSubject
{
    private $observers = array();

    function __construct()
    {
        $this->attach(new XMLObserver);
        parent::__construct();
    }

    function attach(SplObserver $observer)
    {
        $id = spl_object_hash($observer);
        $this->observers[$id] = $observer;
    }

    function detach(SplObserver $observer)
    {
        $id = spl_object_hash($observer);
        unset($this->observers[$id]);
    }

    function notify()
    {
        foreach ($this->observers as $observer) {
            $observer->update($this);
        }
    }

    function save()
    {
       if (in_array($this->changed, array(\'name\')) {
         $this->notify();
       }
       parent::save();
    }
}

class XMLObserver implements SplObserver
{
    function update(SplSubject $subject)
    {
        // code here to update memcached key
    }
}