Core Data Inverse Relationship Not Being Set

Question

I've got two Entities that I'll call A and B. They are configured with To-Many relationships in both directions, so A.myBs and B.myAs are both NSSets.

Here is my bizarre problem.

When I add a B to my A entity, I do it using the mutableSetValueForKey like this:

NSMutableSet *myBSet = [myA mutableSetValueForKey:@"myBs"];
[myBSet addObject:theBtoAdd];

This does add the theBtoAdd to the A entity but does not add the inverse relationship. Core Data context save doesn't kick any errors, but my A object doesn't have the B inverse set. If I exit the application, even the partial relationship isn't saved.

Here's the strange part... if I just switch my code around and do the opposite (there are reasons why this is harder to do for my particular application) - add A to B instead of adding B to A like this:

NSMutableSet *myASet = [myB mutableSetValueForKey:@"myAs"];
[myASet addObject:theAtoAdd];

It works just fine. By the way, I have plenty of other to-many relationships that work. Just this one doesn't.

Couple of other things: 1) My core data object model looks good, but this is the first new entity that I've added under Xcode 4 2) I've check, rechecked and gone blind looking at my custom NSManagedObjects, but they look fine - declared dynamic, NSSet, no conflicting setters/getters... etc.

Any help or debugging suggestions would be greatly appreciated. Thank you!

Answer 1

I just had the same problem, which was why I saw this question. Maybe my solution will help some people in the same situation.

For me the answer was that in the NSManagedObject subclass I had over-riden:

- (void)willChangeValueForKey:(NSString *)inKey withSetMutation:(NSKeyValueSetMutationKind)inMutationKind usingObjects:(NSSet *)inObjects

- (void)didChangeValueForKey:(NSString *)inKey withSetMutation:(NSKeyValueSetMutationKind)inMutationKind usingObjects:(NSSet *)inObjects

This prevents the correct notifications being sent to the parent class (NSManagedObject) which handles the automatic reverse relationships.

Answer 2

I found the error, if you are overriding any of these methods

// KVO change notification
- (void)willChangeValueForKey:(NSString *)key;
- (void)didChangeValueForKey:(NSString *)key;
- (void)willChangeValueForKey:(NSString *)inKey withSetMutation:(NSKeyValueSetMutationKind)inMutationKind usingObjects:(NSSet *)inObjects;
- (void)didChangeValueForKey:(NSString *)inKey withSetMutation:(NSKeyValueSetMutationKind)inMutationKind usingObjects:(NSSet *)inObjects;

it will cause NSManagedObject don't work well and the relationships won't be set correctly

Apple documentation: You must not override these methods.