title: Core Data dependent property on to-many relationship
body: <p>I'm trying to update a custom property in an NSManagedObject whenever an item that it has a to-many relation to changes.  I've overridden the following methods as prescribed by apple:</p>
  
  <pre><code>- (void)addDevicesObject:(Device *)value;
  - (void)removeDevicesObject:(Device *)value;
  - (void)addDevices:(NSSet *)value;
  - (void)removeDevices:(NSSet *)value;
  </code></pre>
  
  <p>and inside the implementation I add or remove observers on the changed objects.  The problem is my override methods are not called when my bindings based UI makes changes to the data.  How should I go about doing this?</p>