带有分组的核心数据 NSFetchRequest答案

【问题标题】：Core Data NSFetchRequest with grouping带有分组的核心数据 NSFetchRequest
【发布时间】：2013-06-12 19:26:21
【问题描述】：

我有一个 fetch 请求，它应该生成一个 tableview，其中包含两个部分。

我的模特：

@interface Player : NSManagedObject

@property (nonatomic, retain) NSString * name;
@property (nonatomic, retain) NSNumber * isFacebookFriend;

此模型上的获取请求应导致一个部分中的人是 isFacebookFriend == YES，而 isFacebookFriend == NO 在第二部分中。

我试过了

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
HBAppDelegate *appDelegate = [UIApplication sharedApplication].delegate;
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Player" inManagedObjectContext:appDelegate.managedObjectContext];
[fetchRequest setEntity:entity];
NSSortDescriptor *nameDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:NO];
[fetchRequest setSortDescriptors:@[nameDescriptor]];
[fetchRequest setResultType:NSDictionaryResultType];
[fetchRequest setPropertiesToGroupBy:@[@"isFacebookFriend"]];
_fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:appDelegate.managedObjectContext sectionNameKeyPath:nil cacheName:@"playerCache"];
NSError *error;
[_fetchedResultsController performFetch:&error];

但这并没有做到。错误是：

2013-06-12 12:27:28.364 TwentyQuestions[25015:c07] Unbalanced calls to begin/end appearance transitions for <UINavigationController: 0xb676aa0>.
2013-06-12 12:27:31.119 TwentyQuestions[25015:c07] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'SELECT clauses in queries with GROUP BY components can only contain properties named in the GROUP BY or aggregate functions ((<NSAttributeDescription: 0xc382320>), name hasRegisteredForGame, isOptional 1, isTransient 0, entity Player, renamingIdentifier hasRegisteredForGame, validation predicates (
), warnings (
), versionHashModifier (null)
 userInfo {
}, attributeType 800 , attributeValueClassName NSNumber, defaultValue (null) is not in the GROUP BY)'

【问题讨论】：

也许显示你完整的获取请求代码？
已添加。只是从核心数据中获取标准

标签： ios core-data nsfetchrequest

【解决方案1】：

要创建带有部分的表格视图，您必须使用sectionNameKeyPath 参数的NSFetchedResultsController。您还必须添加一个（第一个）排序描述符来排序根据段名键路径。

NSSortDescriptor *nameDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:NO];
NSSortDescriptor *isFacebookFriendDescriptor = [[NSSortDescriptor alloc] initWithKey:@"isFacebookFriend" ascending:NO];
[fetchRequest setSortDescriptors:@[isFacebookFriendDescriptor, nameDescriptor]];
_fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
            managedObjectContext:appDelegate.managedObjectContext
              sectionNameKeyPath:@"isFacebookFriend"
                       cacheName:@"playerCache"];

你不必设置setPropertiesToGroupBy，我不建议设置 NSDictionaryResultType 因为这会禁用自动更新通知。

【讨论】：