iOS Components and Frameworks: Understanding the Advanced Features of the iOS SDK

Managed Object Model Migrations

If an object model ever needs to change (even just adding one attribute to an entity is considered a change to the model), model versioning will need to be handled. When loading an existing persistent store, Core Data will check it against the object model to make sure it matches, using a hash calculated on the entities and attributes to determine what constitutes a unique object model. If it does not match, data from the persistent store will need to be migrated to a new persistent store based on the new object model. Core Data can handle many simple migrations automatically with options set in the persistent store coordinator (described later in the chapter), but if any logic is required, a migration might need ...