核心数据从B.cObjects为零的A对象获取B对象

Question

I have a core data object A.

Object A contains an array of objects B - like A.bObjects . Object B contains an array of objects C - like B.cObjects .

How can I get all objects B from object A where B.cObjects is nil?

I can work around this like:

NSMutableArray *array = [[NSMutableArray alloc] init];
for(B *b in A.bObjects)
{
    if([b.cObjects count] == 0)
        [array addObject:b];
}

But i think it will take a lot of resources if I would have a lot of objects B. And I do not like my workaround.:)

Any ideas?

Answer 1

I believe you could use, if you just want all the Bs from a particular A that have empty cObjects.

NSPredicate* predicate = [NSPredicate predicateWithString:@"cObjects.@count == 0"];
NSArray* emptyCObjects = [A.bObjects filteredArrayUsingPredicate:predicate];

If you want all the Bs from all of the As that have empty cObjects, combine that with a NSFetchRequest.

Not a 100% sure which would be more effective, but if I were to hazard a guess, I would say use predicates and KVC as I believe Apple has made some optimisations under the hood. Also its fewer lines of code whi