Core Data - How to establish relationship between two objects in different contexts

In my app, I have the following CoreData model : a Foo has many Bar Entities : Foo <---->> Bar.

To add a new Foo entity, I create a new MOC in which I create a new instance of Foo. This displays the AddFooViewController. Here, I can either Cancel or Save or Add a new Bar entity for this created Foo entity. Problem happens when I add a new Bar entity.

Let me explain in detail : the new Bar entity is created in a totally new MOC. Then, when I decide to save the Bar entity, I try to merge both "Add Bar MOC" and "Add Foo MOC". This should be done by this code :

- (IBAction)addNewBar {
    BarAddViewController *addViewController = [[BarAddViewController alloc] initWithStyle:UITableViewStyleGrouped];

    NSManagedObjectContext *addingContext = [[NSManagedObjectContext alloc] init];
    self.addingManagedObjectContext = addingContext;
    [addingContext release];


    [addingManagedObjectContext setPersistentStoreCoordinator:[self.managedObjectContext persistentStoreCoordinator]];

    // Create new bar
    Bar *bar = (Bar *)[NSEntityDescription insertNewObjectForEntityForName:@"Bar"                                                                      inManagedObjectContext:self.addingManagedObjectContext];
    bar.creationDate = [NSDate date];


    addViewController.delegate = self;
    addViewController.bar = bar;

    UINavigationController *navController = [[UINavigationController alloc] initWithRootViewController:addViewController];
    [self.navigationController presentModalViewController:navController animated:YES];

    [addViewController release];
    [navController release];
}

// Delegate method :
- (void)barAddViewController:(BarAddViewController *)controller didFinishWithSave:(BOOL)save {    
    // Dismiss the modal view to return to the main list
    [self dismissModalViewControllerAnimated:YES];

    // Save modifications
    if (save) {
        NSNotificationCenter *dnc = [NSNotificationCenter defaultCenter];
        [dnc addObserver:self
                selector:@selector(addBarControllerContextDidSave:) 
                    name:NSManagedObjectContextDidSaveNotification 
                  object:addingManagedObjectContext];

        // Assign relationship here ???? foo.bars = bar;

        NSError *error;
        if (![addingManagedObjectContext save:&error]) {
            // Update to handle the error appropriately.
            NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
            exit(-1);  // Fail
        }

        [dnc removeObserver:self 
                开发者_如何学JAVA       name:NSManagedObjectContextDidSaveNotification 
                     object:addingManagedObjectContext];
    } else {
        // Remove bar from addingManagedObjectContext
        [self.addingManagedObjectContext deleteObject:controller.bar];

        NSError *error;
        if (![addingManagedObjectContext save:&error]) {
            // Update to handle the error appropriately.
            NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
            exit(-1);  // Fail
        }
    }

    // Release the adding managed object context.
    self.addingManagedObjectContext = nil;
}

- (void)addBarControllerContextDidSave:(NSNotification*)saveNotification {   
    [self.managedObjectContext mergeChangesFromContextDidSaveNotification:saveNotification];
}

My problem is that I don't know where to establish the relationship between the Foo and the Bar. I know I can only establish the relationship between two NSManagedObject that are in the same context. Does somebody could help me, please ?

Thanks in advance

You need to load both entities in the same managed context to create any sort of relationship (though I guess you've already figures that out!).

You have a Foo from the first managed object context - call it Foo1.

Foo *foo1 = {your initial Foo};

// Get a foo from the new managed context
NSManagedObjectID fooID = [foo1 managedObjectID];
Foo *foo2 = [addingManagedObjectContext objectWithID:fooID];

You can now use Foo2 to create the relationship