Iphone 使用NSFetchedResultsController时节组不一致

我正在使用一个NSFetchedResultsController，它的fetchRequest有一个谓词。然而，每次执行查询时，它似乎并没有给我一致的分组

我已经为NSFetchedResultsController设置了“sectionNameKeyPath”，并根据在运行获取之前是否使用根对象返回了不同数量的节。有时我得到3个部分，其他时间，它返回1个部分，这是预期的结果

如何创建FetchRequestController：

// Create the fetch request for the entity.
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

// Configure the request's entity and its predicate.        
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Employee"
inManagedObjectContext:context];
[fetchRequest setEntity:entity];

// The predicate to find all employees associated with a Group
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"ANY SELF.groups IN %@",
                         [division groups]];

[fetchRequest setPredicate:predicate];

// Sort based on create date and time
NSSortDescriptor *createDateSortDcptor = [[NSSortDescriptor alloc] initWithKey:@"createDateTime" ascending:YES];

NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:createDateSortDcptor, nil];

[fetchRequest setSortDescriptors:sortDescriptors];

// should be grouped by the 'Group' employee belongs to.
NSFetchedResultsController *controller = 
  [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest 
      managedObjectContext:context 
      sectionNameKeyPath:@"groups" 
      cacheName:@"Root"];                   

我的对象模型与另一个问题中概述的相同：

---

有没有办法确保我每次都能得到一致的分组？

事实证明，这样做很简单：

NSFetchedResultsController *controller = 
  [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest 
      managedObjectContext:context 
      sectionNameKeyPath:@"groups.name" 
      cacheName:@"Root"];

我没有意识到我可以在“sectionNameKeyPath”中附加二级属性名