Getting the Minimum Value of an Attribute in a NSSet Made from a One-to-Many Relationship in Core Data

Question

I have several objects set up in Core Data, one of which is "Deck" and one of which is "Card". "Cards" have several numbered relationships, including "id". "Deck" has a one-to-many relationship with cards.

What's the best way to find the Card in a Deck that has the minimum value to some numbered attribute, such as id?

Clearly I can get the list of cards like this:

NSSet *cardList = self.cards;

I think I can build an expression to get a minimum like this:

NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"id"];
NSExpression *minExpression = [NSExpression expressionForFunction:@"min:" 
            arguments:[NSArray arrayWithObject:keyPathExpression]];

But I can't quite suss out how to use that expression to extract the card with the minimum value of id (or just the minimum value of id).

Answer 1

You can do this using Key-Value coding:

//assuming Card has an Id property which is a number
NSNumber *minId = [deck valueForKeyPath:@"cards.@min.Id"];

NSSet *minCards = [[deck cards] filteredSetUsingPredicate:[NSPredicate predicateWithFormat:@"Id==%@", minId]];

will give the NSSet of Cards whose Id property is equal to the minimum in deck.cards.

See the Key-Value Programming Guide's description of Set and Array Operators for more info.