Core Data can “lightly” migrate data

Lightweight data migration is a powerful Core Data tool that allows you to cleanly update your underlying data to match a new data model without needing a mapping model. It only works with basic data changes: adding new attributes, changing a required attribute to an optional one, or making an optional attribute required with a default value. It can also handle limited renaming of attributes, but that gets trickier.

Automatic data migration happens at runtime, which means that your app needs to know that it’s going to happen so that the data can be migrated. You’ll do that in the AppDelegate: