Question

My Cocoa/Application has a Managed Object Context on the main thread.
When I need to update my data my program will:

1. Start a new thread
2. Receive new data from a server
3. Create a new Managed Object Context
4. Send a notification to the main thread in order to merge the two context

This is the function that receive the notification on the main thread

- (void)loadManagedObjectFromNotification:(NSNotification *)saveNotification
{
    if ([NSThread isMainThread]) {
        [self.managedObjectContext mergeChangesFromContextDidSaveNotification:saveNotification];
    } else {
        [self performSelectorOnMainThread:@selector(loadManagedObjectFromNotification:) withObject:saveNotification waitUntilDone:YES];     
    }
}

I do not receive any error.
My problem is the merge result, it actually concatenate Managed Objects from both context.

My Entity are a really simple list of attribute and relationship.

Maybe the merge need some instructions in order to understand when an updated Managed Object IS NOT a new one, but a edited version of the first one.
I imagine that somewhere I need to specify a way to univocally identify an Entity, (an attribute for example can act like an ID) and something like a merge policy (if 2 managed object represent the same object, take the one with the lastModificationDate more recent).

I just need to understand how to correctly merge the 2 contexts in order to have a single updated copy for each object.

UPDATE 1

The problem is now clear to me.
The 2 context has a big difference: the ObjectID.
While the context on the main thread fetched the ManagedObjects with the Persistent Store coordinator, the second thread create those object by fetching a remote URL. Even if the objects have the same contents, they will have 2 different objectID.

My objects had already an unique identificator, I could use setObjectId in order to set this value. (Apple documentation says this is NOT a good idea).

Answer 1

Here is what you need to do in order to correctly merge the contexts.
First, you do not need your own notification. Performing a save operation on a context automatically forwards the following notification to registered observers:

NSManagedObjectContextDidSaveNotification

Therefore, all you need to do is:

1) in your main thread, may be in the viewDidLoad method, register for this notification:

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(contextDidSave:)
                                             name:NSManagedObjectContextDidSaveNotification
                                            object:nil];

2) implement the contextDidSave: method in your main thread as follows:

- (void)contextDidSave:(NSNotification *)notification
{

    SEL selector = @selector(mergeChangesFromContextDidSaveNotification:); 
    [managedObjectContext performSelectorOnMainThread:selector withObject:notification waitUntilDone:YES];

}

3) in your dealloc method add the following:

[[NSNotificationCenter defaultCenter] removeObserver:self];

4) create a new context in your other thread using something like the following method:

- (NSManagedObjectContext*)createNewManagedObjectContext
{

    NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] init]; 
    [moc setPersistentStoreCoordinator:[self persistentStoreCoordinator]];
    [moc setUndoManager:nil];
    return [moc autorelease];
}

5) upon receiving the new data, the proper way to handle this situation is the use of managed object IDs. Since managed object IDs are thread safe, you can pass them from your main thread to the other thread, then use existingObjectWithID:error: to retrieve the object associated to a specific ID, update it and save the context. Now the merge will operate as you expect. Alternatively, if you do not know in advance what managed object IDs must be passed between the threads, then in your other thread you simply fetch the objects using a predicate to retrieve the ones corresponding to the objects retrieved from the server, then you update them and save the context.