Search code examples
core-datanspredicatensset

isSubsetOfSet in NSPredicate string

I have difficulties checking if a nsorderedset is a subset of another nsorderedset in a NSPredicate string.

I can achieve the result with the block below:

request.predicate = [NSPredicate predicateWithBlock:^BOOL(id object, NSDictionary *bindings) {
    NSOrderedSet *biggerSet = [object valueForKey:@"attributeName"];
    return [smallerSet isSubsetOfOrderedSet:biggerSet];
}];

but I believe that in string format could be more performant (am I wrong?).

tried things like @"attribute CONTAINS %@" and @"%@ IN attribute" but with no luck.

Thanks for any help.

Solution

  • Something like this should work:

    NS(Ordered)Set *smallerSet = …
request.predicate =
  [NSPredicate predicateWithFormat:@"SUBQUERY(attribute, $a, $a IN %@).@count == %lu",
         smallerSet, (unsigned long)[smallerSet count]];

    The predicate checks if the number of "attribute" values contained in smallerSet is equal to the number of elements in smallerSet. If yes, then smallerSet must be a subset of the "attribute" set.

    A string based predicate is not generally faster than a block based (compare https://stackoverflow.com/a/21158730/1187415). But a Core Data fetch request (with a SQlite store file) cannot use block based predicates.