为什么CoreData忽略了relationshipKeyPathsForPrefetching？

Question

任何想法为什么这不起作用，我的项目总是错误。

NSManagedObjectContext *tmpContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
tmpContext.parentContext = self.managedObjectContext;

NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Bag"];
request.returnsObjectsAsFaults = NO;
request.relationshipKeyPathsForPrefetching = @[@"items"];

NSArray *bags = [tmpContext executeFetchRequest:request error:nil];

Bag *bag = (Bag *)[bags firstObject];
NSLog(@"%@", bag.items);

我得到......

Relationship 'items' fault on managed object (0x79e90e00)

如果我查看CoreData查询，则其中没有JOIN。

SELECT t0.Z_ENT, t0.Z_PK, t0.Z_OPT, .... FROM ZBAG t0 WHERE t0.Z_ENT = ?