iphone：NSPredicate 与 NSFetchedResultsController 的问题

Question

目前，我的系统中定义了一个“主题”实体：

@interface Topic :  NSManagedObject  
{
}
@property (nonatomic, retain) NSString * path;
@property (nonatomic, retain) NSString * name;
@property (nonatomic, retain) NSNumber * topicID;
@property (nonatomic, retain) NSNumber * parent;
@end

我想使用 NSFetchedResultsController 获取具有特定编号（例如 4001）的主题。我将我的定义为：

- (NSFetchedResultsController *)fetchedResultsController {

    if (_fetchedResultsController != nil) {
        return _fetchedResultsController;
    }

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Topic" inManagedObjectContext:_context];
    [fetchRequest setEntity:entity];

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"topicID == 4100"];
    [fetchRequest setPredicate:predicate];


    NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:@"path" ascending:NO];
    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];

    [fetchRequest setFetchBatchSize:20];

    NSFetchedResultsController *theFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:_context sectionNameKeyPath:nil cacheName:@"Root"];
    self.fetchedResultsController = theFetchedResultsController;
    _fetchedResultsController.delegate = self;

    [fetchRequest release];
    [theFetchedResultsController release];

    return _fetchedResultsController;    

}

..没有谓词，FetchedResultsController 工作正常..但是，当我添加它时，出现错误并且应用程序崩溃..有人可以告诉我我是否错误地定义了 NSPredicate ？

Answer 1

尝试将cacheName设置为nil：

NSFetchedResultsController *theFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:_context sectionNameKeyPath:nil cacheName:nil];

它在类似的情况下帮助了我。

Answer 2

像这样的东西会起作用吗？

[NSPredicate predicateWithFormat:@"topicID == %@", [NSNumber numberWithInt:4100]];