核心数据:多对多关系的预测。(此处不允许使用多个键)

我有两个名为“类别”和“文章”的实体,它们之间有多对多的关系。我想形成一个谓词来搜索category.name等于某个值的所有项目。我有以下几点:

 NSEntityDescription  *entityArticle   = [NSEntityDescription entityForName:@"Article" inManagedObjectContext:managedObjectContext]; 
 NSSortDescriptor  *sortDescriptor   = [[NSSortDescriptor alloc] initWithKey:@"title" ascending:YES];
 NSArray     *sortDescriptors  = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
 NSPredicate    *predicate    = [NSPredicate predicateWithFormat:@"categories.name == [cd] %@", category.name]; 

 [request setSortDescriptors:sortDescriptors];
 [request setEntity:entityArticle];
 [request setPredicate:predicate];

 NSMutableArray *results = [[managedObjectContext executeFetchRequest:request error:nil] mutableCopy];

 if ([results count] > 0)
  NSLog(@"Results found."); 
 else 
  NSLog(@"NO results found."); 

 [request release];
 [sortDescriptor release];
 [sortDescriptors release];

我收到的错误是 *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'to-many key not allowed here'

是否有检索所需数据的选项?