Core Data: error: Exception was caught during Core Data change processing

The line below saved my day:

_managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];

You just have to set the concurrency type as Private. This way it can perform multiple operations on database concurrently on private queue's.

Well, the whole problem was creating the NSManagedObjectContext and everything in the Main Thread and then accessing it or using it in the Background Thread.

So, I have just followed this post and now everything is working perfectly and smooth :)

Thanks a lot for the comments it really put me in the right direction and it was totally what I needed to be able to find the problem..

Thanks!

In the AppDelegate.h

+ (NSManagedObjectContext *)mainQueueContext;
+ (NSManagedObjectContext *)privateQueueContext;

Then, in my AppDelegate.m

#pragma mark - Singleton Access

+ (NSManagedObjectContext *)mainQueueContext
{
    return [self mainQueueContext];
}

+ (NSManagedObjectContext *)privateQueueContext
{
    return [self privateQueueContext];
}

#pragma mark - Getters

- (NSManagedObjectContext *)mainQueueContext
{
    if (!_mainQueueContext) {
        _mainQueueContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
        _mainQueueContext.persistentStoreCoordinator = self.persistentStoreCoordinator;
    }

    return _mainQueueContext;
}

- (NSManagedObjectContext *)privateQueueContext
{
    if (!_privateQueueContext) {
        _privateQueueContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
        _privateQueueContext.persistentStoreCoordinator = self.persistentStoreCoordinator;
    }

    return _privateQueueContext;
}

- (id)init
{
    self = [super init];
    if (self) {
        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(contextDidSavePrivateQueueContext:)
                                                     name:NSManagedObjectContextDidSaveNotification
                                                   object:[self privateQueueContext]];
        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(contextDidSaveMainQueueContext:)
                                                     name:NSManagedObjectContextDidSaveNotification
                                                   object:[self mainQueueContext]];
    }
    return self;
}

#pragma mark - Notifications

- (void)contextDidSavePrivateQueueContext:(NSNotification *)notification
{
    @synchronized(self) {
        [self.mainQueueContext performBlock:^{
            [self.mainQueueContext mergeChangesFromContextDidSaveNotification:notification];
        }];
    }
}

- (void)contextDidSaveMainQueueContext:(NSNotification *)notification
{
    @synchronized(self) {
        [self.privateQueueContext performBlock:^{
            [self.privateQueueContext mergeChangesFromContextDidSaveNotification:notification];
        }];
    }
}

- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

And, finally in my ViewController where I am sending the work to the background..

// dont forget the macro
#define kBgQueue dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)

dispatch_async(kBgQueue, ^{

    id delegate = [[UIApplication sharedApplication] delegate];
    self.managedObjectContext = [delegate privateQueueContext];

    // do something in the background with your managedObjectContext!!!!

 });