If you start from scratch and don't have any data you can use a custom CKRecordID. https://developer.apple.com/documentation/cloudkit/ckrecord/id

Otherwise, you have to check yourself when creating and/updating a record.

If you want to make sure one or more of your custom fields in a record should be unique in your database you have to check yourself.

There are several possibilities to do so:

• You can query for the corresponding field(s) before creating a record and prevent creating.
• You can query for the corresponding field(s) before editing is saved.
• You can prevent editing of the corresponding field(s).