NSManagedObjects created on background turning into faults on main thread - ios

Question

I'm dealing with a strange case. I'm getting some data from a API, transform the response JSON into NSManagedObjects and saving them, all that in a background thread and using a NSPrivateQueueConcurrencyType context whose parent is a NSMainQueueConcurrencyType.
At the same time I'm creating the NSManagedObjects, I'm placing them inside an array to use it in my view controller via a completion block executed on the main therad. Here's a simplified version of what I'm doing.
- (void)getObjectFromAPIWithCompletion:((^)(NSArray *objects, NSError *error))completion
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSManagedObjectContext *backgroundMoc = [[CoreDataStack sharedManager] newChildContextWithConcurrencyType:NSPrivateQueueConcurrencyType];
NSArray *items = [self processToParseResponseFromAPIAndSaveInCoreDataUsingContext:backgroundContext];
[backgroundMoc save:nil];
dispatch_async(dispatch_get_main_queue(), ^{
//Beyond this point, all properties of items are nil or 0 (if integer)
if(completion) completion(items, error);
});
});
}
The weird thing happens inside the completion block. I put a breakpoint on the first line of that block and print the items array on the console. Everything looks fine, with all the items (50 in this case) and their properties are OK.
The problem is when that after that line (where I'm not changing anything on the items array) all objects turn into faults and I'm not getting any data on the properties.
Don't know what is the cause of this rare behaviour. Any idea?
Thanks.
EDIT: Hal's answer put me on the right track so here's what I did to fix this problem. Just before calling the block that will be executed on the main thread I "move" the managed objects from the background context to the main context, fetching with their objectIDs. Like this:
NSArray *objects = //Objects created on background thread with Private queue
NSMutableArray *objectsIDs = [NSMutableArray array];
for (Object *object in object) {
[objectsIDs addObject:object.objectID];
}
//Save on private managed object context and on completion...
[self.managedObjectContext saveWithCompletion:^(NSError *error) {
NSManagedObjectContext *mainMOC = [[CoreDataManager sharedManager] mainContext];
NSMutableArray *fetchedObjects = [NSMutableArray array];
for (NSManagedObjectID *objectID in objectsIDs) {
[fetchedArticles addObject:[mainMOC objectWithID:objectID]];
}
if (completion) completion(fetchedObjects, pagination, nil);
}];
This way, all objects are not faults on the main thread.

Answer 1

If those objects (the contents of items) have been created on the background thread, you can't use them in the main thread. You can't pass NSManagedObject instances between threads.
You'll have to pass NSManagedObjectIDs (which won't be permanent until you've saved the background MOC), or use an NSFetchedResultsController (pointing to the main MOC) on the main thread. This is one of the cases that NSFetchedResultsController was designed for, so I recommend you go that route.