Code pattern / snippet for retrieving single column from cloudkit - ios

Question

I'm building an app which allows users to upload / download info from a common store. I thought cloudKit storage would be ideal for this.
I'd like for users to be able to search records in the store by a KeyWord field, then download the entire record when they select one.
In SQL terms this would be like:
SELECT id,KeyWords from myDB WHERE KeyWords LIKE %searchstring%
Followed by:
SELECT * FROM myDB WHERE id = selectedID
I have been using this code pattern to retrieve records from cloudKit storage:
var publicDatabase: CKDatabase?
let container = CKContainer.defaultContainer()
override func viewDidLoad() {
super.viewDidLoad()
publicDatabase = container.publicCloudDatabase
}
func performQuery(){
let predicate = NSPredicate(value: true)
let query = CKQuery(recordType: "Library", predicate: predicate)
publicDatabase?.performQuery(query, inZoneWithID: nil,
completionHandler: ({results, error in
...
[code to handle results / error here]
...
})
}
but this returns all the content of each record which is a lot of unnecessary info.
I'd like to only fetch a single field from the cloudkit records.
Is there an easy way to do this and does anyone have a code snippet?
CKFetchRecordsOperation allows downloading of restricted columns but requires that you know the ids of the records to download.

Answer 1

For this you can use the desiredKeys property on a CKQueryOperation. For more information see: desiredkeys documentation

Answer 2

OK Got something working. This may well be hacky but I put it up for info for anyone else in the same situation...
let predicate = NSPredicate(value: true) // returns all - replace with whatever condition you want
let query = CKQuery(recordType: "Library", predicate: predicate) // create a query using the predicate
var operation = CKQueryOperation(query: query) // create an operation using the query
operation.desiredKeys = ["KeyWords"] // Array of whatever 'columns' you want to return
// operation.resultsLimit = 15 // optional limit on records
// Define a closure for what to do for each returned record
operation.recordFetchedBlock = { [weak self] (record:CKRecord!) in
// Do whatever you want with each returned record
println(record.objectForKey("KeyWords"))
}
// Define a closure for when the operation is complete
operation.queryCompletionBlock = { [weak self] (cursor:CKQueryCursor!, error:NSError!) in
if cursor != nil {
// returns the point where the operation left off if you want t retrieve the rest of the records
}
dispatch_async(dispatch_get_main_queue(), { () -> Void in
// Do what you want when process complete
self!.tableView.reloadData()
if error != nil {
println("there was an error")
}
})
}
self.publicDatabase!.addOperation(operation) // Perform the operation on given database