NSPredicate for to-many relationship, where a relationship (NSSet) must contain Entities defined in an NSArray

问题

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

So in class A:

@property (nonatomic, retain) NSSet *Bs;

In class B:

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

Assume I have an NSArray *myArray containing (Bj, Bk).

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

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]];

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

回答1:

Does

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

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];

This assumes that the items in myArray are unique.

来源：https://stackoverflow.com/questions/3807532/nspredicate-for-to-many-relationship-where-a-relationship-nsset-must-contain

标签

iphone

objective-c

core-data

nspredicate