NSAsynchronousFetchResult不遵守排序顺序

Question

这是我的获取请求的定义，我使用score属性进行排序：

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:[Product entityName]];
 NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:ProductAttributes.score ascending:YES];
[fetchRequest setSortDescriptors:@[ sortDescriptor ]];

我使用它来定义我的异步提取：

 NSAsynchronousFetchRequest *asynchronousFetchRequest = [[NSAsynchronousFetchRequest alloc] initWithFetchRequest:fetchRequest completionBlock:^(NSAsynchronousFetchResult *result) {
        dispatch_async(dispatch_get_main_queue(), ^{

            // Process Asynchronous Fetch Result
            [weakSelf processAsynchronousFetchResult:result];
        });
    }];

和perfenter代码在manged对象上下文中提取：

// Execute Asynchronous Fetch Request

    [moc performBlock:^{
        // Execute Asynchronous Fetch Request
        NSError *asynchronousFetchRequestError = nil;
        NSAsynchronousFetchResult *asynchronousFetchResult = (NSAsynchronousFetchResult *)[moc executeRequest:asynchronousFetchRequest error:&asynchronousFetchRequestError];

        if (asynchronousFetchRequestError) {
            NSLog(@"Unable to execute asynchronous fetch result.");
            NSLog(@"%@, %@", asynchronousFetchRequestError, asynchronousFetchRequestError.localizedDescription);
        }
    }];

我把它放在我的完成功能中，它大部分时间都失败了。排序有效但有时它在集合中被1或2关闭。假设n项的得分从0到n：

- (void)processAsynchronousFetchResult:(NSAsynchronousFetchResult *)asynchronousFetchResult {

        if (asynchronousFetchResult.finalResult) {

          [_items addObjectsFromArray:asynchronousFetchResult.finalResult];

          for(int i=0 ; i<_items.count ; i++) {            
             Product *product = _items[i];
             assert([product.score intValue]==i);
         }
       }
}

与普通获取结果相同的控制器可以很好地进行排序。任何想法为什么异步提取打破了排序，并有一种方法来强制执行。