NSUndoManager undo Not Working With Core Data

I am trying to create an iPhone application where the user can add entries. When he presses a new entry, a box will popup asking him for some information. Then he can either press “Cancel” or “Save” to discard the data or save it to disk.

For saving, I am using the Core Data framework, which works pretty well. However, I cannot get the “Cancel” button to work. When the window pops up, asking for information, I create a new object in the managed object context (MOC). Then when the user presses cancel, I try to use the NSUndoManager belonging to the MOC.

I would also like to do it using nested undo groups, because there might be nested groups.

To test this, I wrote a simple application. The application is just the “Window based application” template with Core Data enabled. For the Core Data model, I create a single entity called “Entity” with integer attribute “x”. Then inside the applicationDidFinishLaunching, I add this code:

- (void)applicationDidFinishLaunching:(UIApplication *)application {    

  // Override point for customization after app launch    

  unsigned int x=arc4random()%1000;
  [self.managedObjectContext processPendingChanges];
  [self.managedObjectContext.undoManager beginUndoGrouping];

  NSManagedObject *entity=[NSEntityDescription insertNewObjectForEntityForName:@"Entity" 
  [entity setValue:[NSNumber numberWithInt:x] forKey:@"x"];
  NSLog(@"Insert Value %d",x);

  [self.managedObjectContext processPendingChanges];
  [self.managedObjectContext.undoManager endUndoGrouping];
  [self.managedObjectContext.undoManager undoNestedGroup];

  NSFetchRequest *fetchRequest=[[NSFetchRequest alloc] init];
  NSEntityDescription *entityEntity=[NSEntityDescription entityForName:@"Entity"
  [fetchRequest setEntity:entityEntity];
  NSArray *result=[self.managedObjectContext executeFetchRequest:fetchRequest error:nil];
  for(entity in result) {
    NSLog(@"FETCHED ENTITY %d",[[entity valueForKey:@"x"] intValue]);

    [window makeKeyAndVisible];

The idea is simple. Try to insert a new Entity object, undo it, fetch all Entity objects in the MOC and print them out. If everything worked correctly, there should be no objects at the end.

However, I get this output:

[Session started at 2010-02-20 13:41:49 -0800.]
2010-02-20 13:41:51.695 Untitledundotes[7373:20b] Insert Value 136
2010-02-20 13:41:51.715 Untitledundotes[7373:20b] FETCHED ENTITY 136

As you can see, the object is present in the MOC after I try to undo its creation.
Any suggestions as to what I am doing wrong?

NSUndoManager undo Not Working With Core Data