Core Data SUBQUERY和NSFetchedResultsController的Keypath错误 [英] Keypath error with Core Data SUBQUERY and NSFetchedResultsController

问题描述

抱歉，如果这是重复，20分钟的搜索没有产生这种确切的情况或解决方案。

Apologies if this is a duplicate, 20 minutes of searching didn't yield this exact situation or a solution.

我有一个Core数据堆栈， code> XClass ， YClass 和 ZClass 。 XClass 与 YClass 具有一对多的关系。 YClass 与 ZClass 有一对多的关系。

I have a Core Data stack with three classes XClass, YClass, and ZClass. XClass has a one-to-many relationship with YClass. YClass has a one-to-many relationship with ZClass.

使用 NSFetchedResultsController 的实例，我试图获取所有 XClass 的实例，其中至少有1个 YClass 至少有一个 ZClass 。

Using an instance of NSFetchedResultsController, I'm trying to fetch all instances of XClass for which at least 1 YClass has at least 1 ZClass.

定义如下：

// ...stuff
NSFetchRequest * fetchRequest = [NSFetchRequest new];

NSEntityDescription * entity = [NSEntityDescription entityForName:NSStringFromClass([XClass class])
                                           inManagedObjectContext:managedObjectContext];
fetchRequest.entity = entity;

fetchRequest.predicate = 
[NSPredicate predicateWithFormat:@"0 < SUBQUERY(yObjects, $y, $y.zObjects.@count > 0).@count"];

// ..instantiate NSFetchedResultsController and perform fetch

异常与消息：终止应用程序由于未捕获异常'NSInvalidArgumentException'，原因：包含KVC的键集合聚合其中不应该有一个;未能处理$ y.zObjects。@ count'

在其他地方，我已成功获取 YObject 实例与谓词 [NSPredicate predicateWithFormat：@zObjects。@ count> 0];

Elsewhere, I've successfully fetched YObject instances with the predicate [NSPredicate predicateWithFormat:@"zObjects.@count > 0"];

有人可以指点我的错误吗？非常感谢。

Can someone point me to what I'm doing wrong? Thank you much.

推荐答案

在SUBQUERY的谓词字符串中使用集合运算符时，NSPredicate对象此外，您似乎无法对子查询谓词中的集合进行操作。这是除非变量表示谓词中的集合。

It seems that NSPredicate objects violently when you use collection operators in the predicate string for the SUBQUERY. It also appears that you cannot operate on a collection in the subquery predicate. That is unless the variable represents a collection in the predicate.

BAD： SUBQUERY（yObjects，$ y，$ y.zObjects == NIL）> 0

OK： SUBQUERY（yObjects，$ y，SUBQUERY（$ y.zObjects $ z，$ z = = NIL）> 0） 0

以下表达式将所有非nil对象添加到SUBQUERY返回的过滤集合。换句话说，返回的集合将包含具有ZClass实例化的XClass的所有实例。

The following expression will add all non-nil objects to the filtered collection returned by the SUBQUERY. In other words, the returned collection will contain all instances of XClass with instantiations of ZClass.

[NSPredicate predicateWithFormat:@"yObjects.@count > 0 AND (SUBQUERY(yObjects, $y, (SUBQUERY($y.zObjects, $z, $z != NIL).@count > 0)).count > 0)"];

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