CoreData: “Dangling reference to an invalid object.” error

Question

I\'m working on a Cocoa-Touch app, it uses CoreData and has some NSPersistentObject subclasses generated by the XCode model editor.

I\'ve noticed that recently, when

Answer 1

I have another example of how to cause this problem: I have a MOC with a concurrency type of NSMainQueueConcurrencyType. Somewhere in the code I do this:

__block MyObjectType1 *obj1;
[managedObjectContext performBlockAndWait:^{
    obj1 = [NSEntityDescription insertNewObjectForEntityForName:@"Thing" inManagedObjectContext:managedObjectContext];
}];
// some other stuff
[self saveContext];
__block NSManagedObjectID *object1ID;
[managedObjectContext performBlockAndWait:^{
    object1ID = [obj1 objectID];
}];

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // do some slow-ish stuff
    [managedObjectContext performBlockAndWait:^{
        // create new object that has a relationship
        NSManagedObject *obj1_copy = [managedObjectContext objectWithID:object1ID];
        MyObjectType2 *obj2 = [NSEntityDescription insertNewObjectForEntityForName:@"OtherThing" inManagedObjectContext:managedObjectContext];
        obj2.relatedThing = obj1_copy;
    }];
    [self saveContext];
});

It turns out that sometimes, this fails. I still don't understand why, but forcing to get a non-temporary objectID seems to do the trick:

[context performBlockAndWait:^{
    NSError *error;
    [managedObjectContext obtainPermanentIDsForObjects:@[obj1] error:&error];
    object1ID = obj1.objectID;
}];