NSPredicate for to-many关系，其中关系（NSSet）必须包含在NSArray中定义的实体 [英] NSPredicate for to-many relationship, where a relationship (NSSet) must contain Entities defined in an NSArray

问题描述

给定以下核心数据模型：

Given the following Core Data Model:

-> : to-one releationship  
->>: to-many relationship

Entity A ->> Entity B 
Entity B -> A // each B belongs to exactly one A  
Entity B ->> B // a B can be related to other B's

所以在类 A ：

@property (nonatomic, retain) NSSet *Bs;

在类 B ：

@property (nonatomic, retain) A *A;  
@property (nonatomic, retain) NSSet *Bs; 

假设我有一个 NSArray * myArray （Bj，Bk）。

我想要的是获取属于的所有 B Ax ，并与 Bj 和 Bk ：

What I want is to fetch all B's which belong to Ax and are related to Bj and Bk:

下面的代码工作，但是完全丑陋，因为我必须硬编码的NSPredicate基于数字（这里是2，但可能是10左右）的 B 在数组中：

The code below works, but is completely ugly, since I have to hardcode the NSPredicate based on the number (here it's 2, but it could be 10 or so) of B's in the array:

NSEntityDescription *fetchedEntity = [NSEntityDescription entityForName:@"B" inManagedObjectContext:self.managedObjectContext];
[NSPredicate predicateWithFormat:@"A == %@ and (Bs contains %@) and (Bs contains %@)", Ax, [myArray objectAtIndex:0], [myArray objectAtIndex:1]];

如何重写谓词以使其更通用？必须非常明显，但我只是看不到它...

How can I rewrite the predicate so that it is more generic? It must be so obvious, but I just don't see it...

推荐答案

是否

[NSPredicate predicateWithFormat:@"A==%@ AND (ALL %@ IN Bs)", Ax, myArray];

做你期望的？我不确定（和远离我的OS X框）这个谓词是否可以翻译为SQL的Core Data SQLite引擎。您可能需要 SUBQUERY 表达式：

do what you expect? I am not sure (and away from my OS X box) whether this predicate is translatable to SQL for the Core Data SQLite engine. You may need a SUBQUERY expression:

[NSPredicate preidcateWithFormat:@"A==%@ and SUBQUERY(Bs, $x, $x IN %@).@count == %d", Ax, myArray, myArray.count];

这假设 myArray 中的项目唯一。

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