Fetch only specific fields in cloudkit?

Question

I want to fetch only Song_Name field in Musics records in CloudKit. I'm trying below code but It still fetches all the fields in Music record. I think I need to compile operation with publicDB.add(operation) method but It doesn't allow me to declare ¨results¨ as I do now with publicDB.perform(query, in....)

 let predicate = NSPredicate(value: true)
    let query = CKQuery(recordType: "Musics", predicate: predicate)
    let operation = CKQueryOperation(query: query)
    operation.desiredKeys = ["Song_Name"]

    publicDB.perform(query, inZoneWith: nil) { [unowned self] results, error in

        guard error == nil else {
            DispatchQueue.main.async {
                self.delegate?.errorUpdating(error! as NSError)
                print("Cloud Query Error - Refresh: \(error)")
            }
            return
        }
        self.items_music.removeAll(keepingCapacity: true)
        for record in results! {
            let music = Musics(record: record, database: self.publicDB)
            self.items_music.append(music)
        }
        DispatchQueue.main.async {
            self.delegate?.modelUpdated()
        }
    }
}

Answer 1

You are creating a CKQueryOperation and setting the operation's desiredKeys property, but then you are not using this operation. You simply perform a query against the database, so none of the query operation's properties will be used.

You need to assign a record fetched block and query completion block to your operation and then add the operation to the database to actually run it.

let predicate = NSPredicate(value: true)
let query = CKQuery(recordType: "Musics", predicate: predicate)
let operation = CKQueryOperation(query: query)
operation.desiredKeys = ["Song_Name"]
var newItems = [Musics]()
operation.queryCompletionBlock = ( { (cursor, error)->Void in
    guard error == nil else {

        DispatchQueue.main.async {
            self.delegate?.errorUpdating(error! as NSError)
            print("Cloud Query Error - Refresh: \(error)")
        }
        return
    }
    self.items_music = newItems
    DispatchQueue.main.async {
        self.delegate?.modelUpdated()
    }
})

operation.recordFetchedBlock = ( { (record) -> Void in
    let music = Musics(record: record, database: self.publicDB)
    newItems.append(music)
})

publicDB.add(operation)

I have omitted some code that you will need that checks the cursor provided to the completion block. If this isn't nil then you need to issue another query to fetch additional items