如何使用NSFetchedResultsController设置NSPredicate以筛选指定组中的项

我有一个组实体,其中有许多项目实体。一个项目实体可以在多个组中。我的模型的相关部分如下所示:

当我打电话的时候

[fetchedResultsController performFetch:&error]

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'to-many key not allowed here' ***

[NSPredicate predicateWithFormat:@"groups == %@", self.group];

但我不知道我做错了什么。我已经阅读了NSPredicate文档并尝试了以下方法:

[NSPredicate predicateWithFormat:@"ANY groups IN %@", [NSArray arrayWithObject:self.group]];
[NSPredicate predicateWithFormat:@"ANY groups LIKE %@", self.group];
[NSPredicate predicateWithFormat:@"ANY groups == %@", self.group];
[NSPredicate predicateWithFormat:@"ANY groups IN %@", [NSArray arrayWithObject:self.group]];
[NSPredicate predicateWithFormat:@"groups == %@", self.group];

我有一个NSFetchedResultsController,我正试图将其配置为仅显示特定组中的项。设置NSFetchedResultsController的代码如下所示:

// create the fetch request
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

// configure the entity
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Item" inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:entity];

// configure the predicate
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"groups == %@", self.group];
[fetchRequest setPredicate:predicate];

// configure the sort descriptors
NSSortDescriptor *indexSortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"uniqueID" ascending:YES];
NSArray *sortDescriptors = [NSArray arrayWithObject:indexSortDescriptor];
[fetchRequest setSortDescriptors:sortDescriptors];

// create the fetched results controller
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:@"ItemsCache"];

// configure the fetched results controller 
aFetchedResultsController.delegate = self;