Custom Entity Migration Policies

When Core Data migrates data between model versions, it uses a series of different objects to handle entity and property mappings. One of these objects is an NSEntityMigrationPolicy object.

An entity migration policy object determines how a source entity will be migrated to a destination entity; in order to derive and set the yearOfBirth attribute, you’ll need to specify a custom subclass of NSEntityMigrationPolicy for the Person entity to define the custom behavior.