Staged migrations
Migrate complex data models containing changes that are incompatible with lightweight migrations.
Overview
Core Data uses lightweight migrations to keep the data in your app’s persistent store consistent with the app’s managed object model. A lightweight migration automatically determines the differences between two model versions and generates a mapping model that Core Data then uses to make the necessary changes to the persistent store. Lightweight migrations support a number of common operations, such as adding an entity, removing a relationship, changing a nonoptional attribute to an optional attribute, renaming an entity, and so on.
As your object model evolves, you may find that the aggregate changes between two model versions exceed the capabilities of lightweight migrations. For example, if you change an optional attribute to be nonoptional, there’s no way for a lightweight migration to infer the default value it needs to assign to any instances of that attribute with a nil value.
Staged lightweight migrations solve this problem by reducing an incompatible migration into a series of compatible stages. A migration manager runs these stages in a specific order, providing opportunities for you to prepare the persistent store before each stage runs, and perform any cleanup afterward. This enables you to handle scenarios like changing an optional attribute to be nonoptional because you have an opportunity to set any nil values to a concrete value before the stage runs.