Saving Modified Data in CloudKit

Question

I have been testing out CloudKit as i wish to release an app using it when the release of iOS8 occurs. It seems simple enough to save data using the code below:

CKRecordID * recordID = [[CKRecordID alloc] initWithRecordName:@"basicRecord"];
CKRecord * record = [[CKRecord alloc] initWithRecordType:@"basicRecordType" recordID:recordID];
[record setValue:@"defaultValue" forKey:@"defaultKey"];
CKDatabase *database = [[CKContainer defaultContainer] publicCloudDatabase];
[database saveRecord:record completionHandler:^(CKRecord *record, NSError *error) {

    if (error) {
        NSLog(@"Error: %@", error);
    } else {
        NSLog(@"Record Saved!");
    }
}];

and I receive no errors from this. However, if i try to run the code again, maybe because i have changed the record value to

[record setValue:@"newValue" forKey:@"defaultKey"];

I receive an error which begs the question, how do i go about saving a modified piece of data. After all, this is a fundamental part of saving things to the cloud. The error is below and any help would be greatly appreciated, don't hesitate to ask for further information.

Error: <CKError 0x17024afb0: "Server Record Changed" (14/2017); "Error saving record <CKRecordID: 0x144684a80; basicRecord:(_defaultZone:__defaultOwner__)> to server: (null)"; uuid = 182C497F-966C-418A-9E6A-5563BA6CC6CD; container ID = "iCloud.com.yourcompany.CloudKit">

Answer 1

This error is probably because saveRecord: works only for new records or records that are newer than the version on the server:

This method saves the record only if it has never been saved before or if it is newer than the version on the server. You cannot use this method to overwrite newer versions of a record on the server. CKDatabase docs

The recommended approach to modify an existing record (or set of records) is to use a CKModifyRecordsOperation set with the desired savePolicy to deal with conflicts:

After modifying the fields of a record, use this type of operation object to save those changes to a database. (...) When saving records, the value in the savePolicy property determines how to proceed when conflicts are detected on the server. CKModifyRecordsOperation docs