Core Data Objects Section 错误问题

今天在使用NSFetchedResultsController的时候，使用sectionNameKeyPath参数对数组进行分组，得到的结果数据分组却不正确，google到下面一遍文章，原因及解决办法讲得很清楚，翻译过来分享给大家。

原文地址

NSFetchedResultsController是一个非常可爱的类。使用XCode自带的Core Data模板，你可以非常容易的在table view中创建漂亮的Managed Object列表。而增加几行代码，你就可以让NSFetchedResultsController将你的objects按section进行分组。要实现此功能，在构造NSFetchedResultsController对象时指定一个key-path即可。但是这里还需要额外的一步，否则分组将会混淆。

在一个APP中，我创建了一个食物列表，并按食物类型进行分组显示：

使用key-path进行分组的fetchedResultsController函数：

- (NSFetchedResultsController *)fetchedResultsController {
	if (fetchedResultsController != nil) {
		return fetchedResultsController;
	}
 
	// Create and configure a fetch request with the food entity.
	NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
	NSEntityDescription *entity = [NSEntityDescription entityForName:@"RWFood" inManagedObjectContext:managedObjectContext];
	[fetchRequest setEntity:entity];
 
	// Create the sort descriptors array.
	NSSortDescriptor *nameDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];
	NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:nameDescriptor, nil];
	[fetchRequest setSortDescriptors:sortDescriptors];
 
	// Create and initialize the fetch results controller.
	NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:@"category" cacheName:@"Food"];
	self.fetchedResultsController = aFetchedResultsController;
	fetchedResultsController.delegate = self;
 
	// Memory management.
	[aFetchedResultsController release];
	[fetchRequest release];
	[nameDescriptor release];
	[sortDescriptors release];
 
	return fetchedResultsController;
}

连续保存并退出程序多次，你会发现某些列表项（食物）显示在了错误的分类中。如果你够细心的话，你会发现列表项实际上是按升序进行显示了。查看代码，我们确实是这样要求程序做的。

根据这篇文章，当你指定了一个key-path用于进行分组时，你必须保证第一个Sort Descriptor是使用此key-path进行排序的。增加一个Sort Descriptor即可解决我们的问题。

增加了另一个Sort Descriptor的fetchedResultsController函数修正版本：

- (NSFetchedResultsController *)fetchedResultsController {
 
	if (fetchedResultsController != nil) {
		return fetchedResultsController;
	}
 
	// Create and configure a fetch request with the plant entity.
	NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
	NSEntityDescription *entity = [NSEntityDescription entityForName:@"RWPlant" inManagedObjectContext:managedObjectContext];
	[fetchRequest setEntity:entity];
 
	// Create the sort descriptors array.
        NSSortDescriptor *typeDescriptor = [[NSSortDescriptor alloc] initWithKey:@"type" ascending:YES];
	NSSortDescriptor *nameDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];
	NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:typeDescriptor, nameDescriptor, nil];
	[fetchRequest setSortDescriptors:sortDescriptors];
 
	// Create and initialize the fetch results controller.
	NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:@"type" cacheName:@"Plants"];
	self.fetchedResultsController = aFetchedResultsController;
	fetchedResultsController.delegate = self;
 
	// Memory management.
	[aFetchedResultsController release];
	[fetchRequest release];
        [categoryDescriptor release];
	[nameDescriptor release];
	[sortDescriptors release];
 
	return fetchedResultsController;
}