Ios 为什么CoreData会忽略RelationshipKeyPaths预处理？

如果你知道为什么这样不行，我的东西总是有毛病

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，其中就没有连接

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

这可能是相关的。