如何创建一个Core Data谓词来测试一个关系是否包含所有给定的对象？ [英] How to create a Core Data predicate to test that a relation contains all given objects?

问题描述

设置：

我有一个与B有一对多关系的核心数据对象A.调用关系items。所以，a.items返回与A相关联的所有B。

I have a Core Data object A that has a to-many relation to B. Call the relation "items". So, a.items returns all B-s associated with A.

现在，我有一个手动组成的B对象的NSSetitemSet。

Now, I have a manually composed NSSet "itemSet" of B objects.

我想要执行以下操作：

return all A objects whose "items" relation exactly matches itemSet

如何构造一个谓词？我试过这个：

How do I construct a predicate for that? I’ve tried this:

NSPredicate *predicate = [NSPredicate predicateWithFormat:
                          @"(ALL items in %@)", itemSet];

但是只是给我不支持的谓词c>。

But that just gives me Unsupported predicate (null).

这：

NSPredicate *predicate = [NSPredicate predicateWithFormat:
                          @"(items in %@)", itemSet];

告诉我谓词未执行的SQL生成。

tells me unimplemented SQL generation for predicate. Interesting, but not helpful.

那么，用集合过滤关系的正确方法是什么？

So what’s the right way to filter the relation with a set?

推荐答案

以下谓词可以工作：

[NSPredicate predicateWithFormat:@"(items.@count == %d) AND (SUBQUERY(items, $x, $x IN %@).@count == %d)",
                      itemSet.count, itemSet, itemSet.count];

谓词首先检查项目数等于给定的 itemSet ，然后检查 itemSet 成员的项目数量是否也等于 itemSet 。如果两者都为真，则项必须等于 itemSet 。

The predicate checks first that the number of items is equal to the size of the given itemSet, and then checks that the number of items which are member of itemSet is also equal to the size of itemSet. If both are true, then items must be equal to itemSet.

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