Question

I’m having problems with duplicate objects of the same entity within a single context when using two managed object contexts.

Consider the following code:

[childMOC performBlockAndWait:^{

    // CREATE PERSON IN CHILD MOC
    Person *person = [NSEntityDescription insertNewObjectForEntityForName:@"Person" 
                                                   inManagedObjectContext:childMOC];

    person.name = @"Ben";

    // SAVE CHILD MOC TO PUSH CHANGES TO MAIN MOC
    [childMOC save:nil];

    NSManagedObjectID *personID = person.objectID;

    [mainMOC performBlockAndWait:^{
        // SAVE MAIN MOC TO PERSIST THE PERSON AND REPLACE ID TO PERMANENT
        [mainMOC save:nil];

        // GET THE PERSON IN THE MAIN MOC
        Person *personInMainContext = (Person*)[mainMOC objectWithID:personID];

        // GET THE PERSON'S NEW PERMANENT ID
        NSManagedObjectID *personIdAfterSaveToPersistentStore = personInMainContext.objectID;

        [childMOC performBlockAndWait:^{

            // GET THE PERSON IN THE CHILD MOC WITH ITS NEW PERMANENT ID
            // (this is common when sending the id from mainMOC to childMOC)
            Person *samePersonFetchedFresh = (Person*)[childMOC objectWithID:personIdAfterSaveToPersistentStore];

            // THE PERSON OBJECTS SHOULD BE EXACTLY THE SAME BECAUSE THE MOC GUARANTEES UNIQUING
            samePersonFetchedFresh.name = @"Jerry";
            NSLog(@"%@ & %@", person.name, samePersonFetchedFresh.name);

            // OUTPUT: Ben & Jerry
            // NOT THE SAME?!
        }];
    }];
}];

This means that an object created in the child MOC loose its uniquing ability when it has been saved in the main MOC / persistent store.

Can anyone explain why uniquing doesn’t work in this situation?

Answer 1

My solution was to:

1) Use the background MOC as the parent MOC and the main MOC as a child. As a bonus I don’t need to save the main MOC to get the permanent IDs.

[DC.backgroundMOC performBlock:^{
    // Add, save and update managed objects
    [DC saveContext]; // The changes is being pushed to the main context
}];

2) Use NSManagedObjectContextDidSaveNotification to keep the main MOC up to date (the main MOC is updating the UI)

- (void) backgroundMOCSaved:(NSNotification*)notification {
    [mainMOC performBlock:^{
        [mainMOC mergeChangesFromContextDidSaveNotification:notification];
    }];
}