Fetch attributes from to many relationship in Swift Core Data

Question

I cannot for the life of me get this to work. I have a 'Match' entity and a 'Players' entity joined by a many-to-many relationship. For each match I am trying to create a string of names that are pulled from the Players entity in Core Data. Here is the latest incarnation of what I have been trying to achieve this with:

let players = matchData[indexPath.row].value(forKeyPath: "players") as? NSManagedObject
    let playerNames = players?.value(forKey: "firstName") as? NSMutableArray

    let playersString = playerNames?.componentsJoined(by: ",")

matchData is the NSManagedObject where my fetched data is held. "players" is the name of the relationship to the "Players" entity. And "firstName" is the attribute that I am trying to string together.

This however returns 'nil' for each match even though I know for sure that there are player records saved for each match. I can display other matchData items fine but am having trouble with this to-many relationship.

jrturton jrturton · Accepted Answer · 2017-04-24T09:25:53

value(forKeyPath: "players") as? NSManagedObject

If players is really a to-many relationship then this code will always return nil, because it will try to cast to an NSManagedObject but the actual type is a Set of NSManagedObjects.

value(forKeyPath: "players") as? Set<NSManagedObject> will get you past the first hurdle.

You'd be doing yourself a favour to specify managed object subclasses as well, then you'll have properties generated for all of these things which will make your code much cleaner and easier to understand.

Fetch attributes from to many relationship in Swift Core Data

1 Answers