NSFetchRequest Controller第一次未更新数据

Question

当我进入PhotoVC时，我第一次没有更新数据 NSFetchRequestController 。比方说，我有摄影师和照片实体，分别是一对多关系“一位摄影师可以有多张照片”，

当我点击摄影师单元格时，它会将我带到photosVC，在后台，我会更新照片实体，但NSFetchResultController不会显示结果。然而，当我回击并回到PhotoVC时，它向我显示数据......不确定为什么 NSFetchResultController 第一次没有更新？

我使用下面提到的FetchResultCOntroller代码

- (NSFetchedResultsController *)fetchedResultController
{
    if (!_fetchedResultController) {

        NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"Photo"];
        fetchRequest.predicate = [NSPredicate predicateWithFormat:@"Photos.PhotographerID = %@",];
        fetchRequest.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"timeStamp" ascending:YES]];
        //[fetchRequest setFetchBatchSize:20];

        _fetchedResultController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
                                                                       managedObjectContext:moc
                                                                         sectionNameKeyPath:nil cacheName:nil];
        _fetchedResultController.delegate = self;

    }
    return _fetchedResultController;
}