Resolving an NSManagedObject conflict with multiple threads, relationships, and pointers

I'm having a conflict when saving a bunch of NSManagedObjects via an outside thread. For starters, I can tell you the following:

1. I'm using a separate MOC for each thread.
2. The MOCs share the sam开发者_如何学Goe persistent store coordinator.
3. It's likely that an outside thread is modifying one or many of the records that I'm saving.

OK, so with that out of the way, here's what I'm doing.

In my outside thread, I'm doing some computation and updating a single value in a bunch of managed objects. I do this by looking up the object in the persistent store by my primary key, modifying the single decimal property, and then calling save on the bunch all at once.

In the meantime, I believe the main thread is doing some updating of its own.

When my outside thread does its big save on its managed object context, I get an exception thrown stating a large number of conflicts. All of the conflicts seem to be centered around a single relationship on each record. Though the managed object in the persistent store and my outside thread share the same ObjectID for this relationship, they don't share the same pointer. Based on what I see, that's the only thing that's different between the objects in my NSMergeConflict debug output.

It makes sense to me why the two objects have relationships with different pointers -- they're in different threads. However, as I understand it from Apple's documentation, the only thing cached when an object is first retrieved from the persistent store are the global IDs. So, one would think that when I run save on the outside thread MOC, it compares the ObjectIDs, sees they're the same, and lets it all through.

So, can anyone tell me why I'm getting a conflict?

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Per the documentation in the Concurrency with Core Data chapter of The Core Data Programming Guide, the recommended configuration is for the contexts to share the same persistent store coordinator, not just the same persistent store.

Also, the section Track Changes in Other Threads Using Notifications of the same chapter states if you're tracking updates with the NSManagedObjectContextDidSaveNotification then you send -mergeChangesFromContextDidSaveNotification to the main thread's context so it can merge the changes. But if you're tracking with NSManagedObjectContextDidChangeNotification then the external thread should send the object IDs of the modified objects to the main thread which will then send -refreshObject:mergeChanges: to its context for each modified object.

And really, you should know if the main thread is also performing updates through its controller, and propagate its changes in like manner but in the opposite direction.

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You need to have all your contexts listening for NSManagedObjectContextDidSaveNotification from any context that makes changes. Otherwise, only the front context will be aware of changes made on the background threads but the background context won't be aware of changes on the front thread.

So, if you have three threads and three context each of which makes changes, all three context must register for notifications from the other two.

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Unfortunately, it seems as though this bug was actually being caused by something else -- I was calling the operation causing the error more than once at the same time when I shouldn't have been. Although this doesn't answer the initial question as to why pointers matter in conflicts, updating my code to prevent this situation has resolved my issue.