CKFetchRecordsOperation + CKQueryOperations ... what am I missing?

Question

Managed to cobble together a CKFetchRecordsOperation after much searching for sample code; and here it is... but I must have missed something. Don't get me wrong it works a treat... but...

To execute a CKFetchRecordsOperation you need an NSArray of CKRecordIDs; to get a NSArray of CKRecordIDs, you need to execute CKQuery thru which you can build your NSArray of CKRecordIDs.

But wait a minute, the process of extracting the CKRecordIDs uses a CKQuery, thru which I could simply download the CKRecords anyway?

How do you get your NSArray of CKRecordIDs if not with a CKQuery?

-(void)doSingleBeaconsDownload
{
    CKDatabase *publicDatabase = [[CKContainer containerWithIdentifier:@"iCloud.cqd.ch.BeaconBrowser"] publicCloudDatabase];
    NSPredicate *predicatex = [NSPredicate predicateWithFormat:@"iBeaconConfig = %@",iBeaconsConfirmed.giReferenceID];
    CKQuery *query = [[CKQuery alloc] initWithRecordType:@"Running" predicate:predicatex];

    [self.delegate performSelectorOnMainThread:@selector(processCompleted:) withObject:@"Downloading Configuration ..." waitUntilDone:YES];

    [publicDatabase performQuery:query inZoneWithID:nil completionHandler:^(NSArray *results, NSError *error) {

        if (error) {
            NSLog(@"Batch Download Error iCloud error %@",error);
        }
        else {
            NSMutableArray *rex2download = [[NSMutableArray alloc] init];
            for (CKRecord *rex in results) {
                [rex2download addObject:rex.recordID];
            }

            CKFetchRecordsOperation *fetchRecordsOperation = [[CKFetchRecordsOperation alloc] initWithRecordIDs:rex2download];

           /* fetchRecordsOperation.perRecordCompletionBlock = ^(CKRecord *record, CKRecordID *recordID, NSError *error) {
                if (error) {
                    // Retain the record IDs for failed fetches
                }
                else {
                   // Do something with each record downloaded
                }
            };*/

            fetchRecordsOperation.perRecordProgressBlock = ^(CKRecordID *record, double recordsDownloaded) {
                if (error) {
                    // damn ...
                } else {
                    NSLog(@"Downloaded %f", recordsDownloaded);
                }
            };
            fetchRecordsOperation.fetchRecordsCompletionBlock = ^(NSDictionary *recordsByRecordID, NSError *error) {
                if (error) {
                    // Failed to fetch all or some of the records
                }
                else {
                    for(CKRecord *record in results) {
                        NSLog(@"Fini download %lu",(unsigned long)[recordsByRecordID count]);
                    }
                    [self.delegate performSelectorOnMainThread:@selector(beaconsDownloaded:) withObject:noOf waitUntilDone:YES];
                }
            };
            [publicDatabase addOperation:fetchRecordsOperation];
        }
    }];
}

Answer 1

From Apple Documentation: A CKFetchRecordsOperation object retrieves CKRecord objects (whose IDs you already know) from iCloud.

A CKQueryOperation is used to retrieve CKRecords from iCloud based on some Query, so you can get them even if you do not know their recordIDs. A CKFetchRecordsOperation is used ONLY when you have the CKRecordIDs. You can create a CKRecordID without accessing iCloud, and you can store them in any local storage you have.

A good use case, which I use for this kind of operation, is when you want to modify a CKRecord, you need to first retrieve it from iCloud (using CKFetchRecordsOperation) and then save it back using CKModifyRecordsOperation.

Have a look at the two WWDC 2014 Videos on CloudKit that explain this pretty well.