New object created in child context doesn't exist in parent context after saving

Question

I inserted an object in child context and got the objectID, then I saved both child context and parent context. now I want to get the object through objectID, but I can only get it with child context but parent context, why?

...
NSManagedObjectContext *childContext = [[[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType] autorelease];
childContext.parentContext = self.parentContext;
[childContext performBlock:^{
    NSManagedObject *obj = [NSEntityDescription insertNewObjectForEntityForName:@"Event" inManagedObjectContext:childContext];

    if (![childContext save:&error]) {
        NSLog(@"%@", error);
    }

    [self.parentContext performBlockAndWait:^{
        if (![self.parentContext save:&error]) {
            NSLog(@"%@", error);
        }
    }];

    NSManagedObjectID *objID = [obj ObjectID];
    NSManagedObject *objTmp = [self.parentContext existingObjectWithID:objID error:&error];
    // Why objTmp is nil here?

    NSManagedObjectID *objIDTmp = [[[self.parentContext executeFetchRequest...] objectatindex:0] objectID];
    // objIDTmp is different from objID 
}];

Eric Hedstrom Eric Hedstrom · Accepted Answer · 2015-04-04T02:21:09

A new object will get a new NSManagedObjectID when it is saved to the permanent store. You can call obtainPermanentIDsForObjects to get a permanent id before saving it.

[childContext obtainPermanentIDsForObjects:@[obj] error:&error];
NSManagedObjectID *objID = [obj ObjectID];

New object created in child context doesn't exist in parent context after saving

2 Answers