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

core-data, nsset, objective-c

Solution

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.

Problem

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).

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