Question

Got a Stumper (at least for me).

I am using iOS 5.0 w/ ARC, and Core Data inside of UIManagedDocument.

I have an Entity (Group) with a to-many relationship (called people) to entity (Person). When I add a new Group, and add then a new Person (setting the person’s .group relationship to the new group), I cannot retrieve the related people using a predicate on the Person entity where (“group == %@”, myGroup). I also tried using the Group’s addPerson setter.

If I shut down XCode simulator and rerun it, it recognizes the relationship that was created in the previous run, I can even add new people to the existing Group object. I just can’t add a new Group and then add people to without shutting down the Simulator (or device if I’m running on device) in order to see the relationship.

If I do a [group.people count], immediately after adding the new Group and a related Person, it gives me the correct number. But a fetch with a predicate doesn’t work until I restart the app.

It seems as if the managedObjectContext of the UIManagedDocument is not seeing the relationship. I’ve tried saving the context, saving context.parentContext, and saving the Document. None of that helped.

Any ideas would be appreciated!

Answer 1

A UIManagedDocument will save, well, basically, when it feels like it; you don’t have control as to when that occurs. It will, however, certainly save on application termination, which is why you see the inserts when you restart.

As a result, while you may think you have permanent object ids, they’re actually likely temporary, and thus can’t be fetched. Simply dumping them via an NSLog would validate this, as temporary object ids show up as such when logged.

To force them to be permanent and thus usable, try the following after performing your additions. Assuming you have a UIManagedDocument as an ivar:

- (void)performUpdate
{
    NSManagedObjectContext * context = self.managedDocument.managedObjectContext;
    NSSet                  * inserts = [context insertedObjects];

    if ([inserts count])
    {
        NSError * error = nil;

        if ([context obtainPermanentIDsForObjects:[inserts allObjects]
                                            error:&error] == NO)
        {
            NSLog(@"BAM! %@", error);
        }
    }

    [self.managedDocument updateChangeCount:UIDocumentChangeDone];
}

Obviously, you’d replace the error handling with something a bit better here. The UIManagedDocument will now save at some point in the future (again, you have absolutely no control over when that’ll happen, we’re just asking it to do so in the last line, in the same way that an undo manager would), but newly-inserted objects should now have usable permanent ids and fetches should work properly.

And yes, this seems a bit odd to me too, but this appears to be the correct way to do things with a UIManagedDocument in play.

Frankly, I’d love for someone to tell me I’m incorrect and provide a better solution.