Why Do Many-To-Many Relationships Disappear When Moving a Core Data Persistent Store?

Question

I have an application that uses Core Data to persist information. I'm adding a Share Extension to my application, which will need to access the same data, so I need to move my persistent store from my app's sandbox into an app group sandbox. As recommended in Update Core Data store location to support App Groups, I'm using a migration to do that. The problem is that my many-to-many relationships disappear during that migration.

I was able to re-create the problem starting with a basic Single View Application with Core Data. A button is wired to create the objects in the default managedObjectContext (which I had to add a queue to), and show a status message:

AppDelegate * delegate = [UIApplication sharedApplication].delegate;

[delegate.managedObjectContext performBlock:^{
    Book * book1 = [[Book alloc] initWithEntity:[NSEntityDescription entityForName:@"Book" inManagedObjectContext:delegate.managedObjectContext] insertIntoManagedObjectContext:delegate.managedObjectContext];

    Book * book2 = [[Book alloc] initWithEntity:[NSEntityDescription entityForName:@"Book" inManagedObjectContext:delegate.managedObjectContext] insertIntoManagedObjectContext:delegate.managedObjectContext];

    Topic * topic1 = [[Topic alloc] initWithEntity:[NSEntityDescription entityForName:@"Topic" inManagedObjectContext:delegate.managedObjectContext] insertIntoManagedObjectContext:delegate.managedObjectContext];

    Topic * topic2 = [[Topic alloc] initWithEntity:[NSEntityDescription entityForName:@"Topic" inManagedObjectContext:delegate.managedObjectContext] insertIntoManagedObjectContext:delegate.managedObjectContext];

    Topic * topic3 = [[Topic alloc] initWithEntity:[NSEntityDescription entityForName:@"Topic" inManagedObjectContext:delegate.managedObjectContext] insertIntoManagedObjectContext:delegate.managedObjectContext];

    book1.title = @"Breadbaker's Apprentice";
    book1.authorName = @"Peter Reinhart";

    book2.title = @"Tartine Bread";
    book2.authorName = @"Chad Robertson";

    topic1.topicName = @"Bread";
    topic1.topicPriority = [NSDecimalNumber decimalNumberWithString:@"0.9"];
    [topic1 addBooksObject:book2];

    topic2.topicName = @"Baking";
    topic2.topicPriority = [NSDecimalNumber decimalNumberWithString:@"0.7"];
    [topic2 addBooksObject:book1];
    [topic2 addBooksObject:book2];

    topic3.topicName = @"San Francisco";
    topic3.topicPriority = [NSDecimalNumber decimalNumberWithString:@"0.2"];
    [topic3 addBooksObject:book2];

    NSError * saveError;
    if(![delegate.managedObjectContext save:&saveError])
    {
        NSLog(@"Failed to save for some reason: %@", saveError.debugDescription);
        dispatch_async(dispatch_get_main_queue(), ^{
            self.statusLabel.text = [NSString stringWithFormat:@"Failed to save for some reason: %@", saveError.debugDescription];
        });
    }
    else
    {
        dispatch_async(dispatch_get_main_queue(), ^{
            self.statusLabel.text = @"OK, added objects";
        });
    }

}];

When run in the simulator I can use sqlite3 on the command line to inspect the SQLite db—and specifically the Z_1TOPICS linking table Core Data creates—after adding those objects:

sqlite> .tables
ZBOOKS        ZTOPICS       Z_1TOPICS     Z_METADATA    Z_PRIMARYKEY
sqlite> SELECT * from Z_1TOPICS;
1|3
1|2
2|2
2|3
2|1

All looks about right. Next, a second button is setup to migrate the persistent store to the App Group directory:

AppDelegate * delegate = [UIApplication sharedApplication].delegate;

NSURL *newStoreURL = [[delegate groupDocumentsDirectory] URLByAppendingPathComponent:@"Migration_Bug_Demonstration.sqlite"];

NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"Migration_Bug_Demonstration" withExtension:@"momd"];

NSManagedObjectModel * managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];

NSPersistentStoreCoordinator *tempCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel: managedObjectModel];

NSURL *oldStoreURL = [[delegate applicationDocumentsDirectory] URLByAppendingPathComponent:@"Migration_Bug_Demonstration.sqlite"];

NSPersistentStore   *sourceStore        = nil;
NSPersistentStore   *destinationStore   = nil;
NSDictionary *RWoptions = @{NSMigratePersistentStoresAutomaticallyOption: @YES,
                            NSInferMappingModelAutomaticallyOption: @YES};
NSDictionary * ROoptions = @{NSReadOnlyPersistentStoreOption:@YES,NSMigratePersistentStoresAutomaticallyOption: @YES,
                             NSInferMappingModelAutomaticallyOption: @YES};
NSError * error;
// Add the source store
if (![tempCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:oldStoreURL options:ROoptions error:&error]){
    // Handle the error
    NSLog(@"Error adding old store: %@ -- %@", error, error.userInfo);
}
else
{
    sourceStore = [tempCoordinator persistentStoreForURL:oldStoreURL];
    if (sourceStore != nil){
        // Perform the migration
        destinationStore = [tempCoordinator migratePersistentStore:sourceStore toURL:newStoreURL options:RWoptions withType:NSSQLiteStoreType error:&error];
        if (destinationStore == nil){
            NSLog(@"Error migrating store: %@ -- %@", error, error.userInfo);
            // Handle the migration error
            dispatch_async(dispatch_get_main_queue(), ^{
                self.statusLabel.text = [NSString stringWithFormat:@"Failed to migrate: %@", error.debugDescription];
            });
        } else {

            dispatch_async(dispatch_get_main_queue(), ^{
                self.statusLabel.text = @"OK, moved store";
            });
        }
    }
}

Much of that code is straight out of the Question linked above. The relationships between Books and Topics are not migrated, but the objects themselves are:

sqlite> .tables
ZBOOKS        ZTOPICS       Z_1TOPICS     Z_METADATA    Z_PRIMARYKEY
sqlite> SELECT * FROM Z_1TOPICS;
sqlite> SELECT * FROM ZTOPICS;
1|2|1|0.2|San Francisco
2|2|1|0.7|Baking
3|2|1|0.9|Bread
sqlite> SELECT * FROM ZBOOKS;
1|1|1|Peter Reinhart|Breadbaker's Apprentice
2|1|1|Chad Robertson|Tartine Bread

Indeed, using the "-com.apple.CoreData.SQLDebug 1" launch argument before running all this, I can verify that Core Data is writing to the ZTOPICS and ZBOOKS tables, but never to the Z_1TOPICS table.

So now I'm stuck. I'm not even sure what I else I can do to investigate this problem—maybe perform a custom migration so I can watch (or even manually perform) the relationship migration? I might just try to move the persistent store files manually (even though that's a No-No), because I'm not sure what else I can do. Thanks for your time.