Core Data Migration: Do I Need a new Mapping Model for each new Model Version I Add?

Question

I've done a custom Core Data migration several versions back when doing some structure changes in my app. (So created a new model version, and a mapping model with custom policy class).

Now, I want to do some more changes. So I've created another model version. Now, I'm not sure whether I need to create another mapping model for this change? If I do, will core data just figure out the appropriate one to use based on the users version?

Will I also need to create another custom policy class, or can I somehow add the new logic to the first one?

Lastly, will I need to add any logic for migrating from the original database straight to the current database? Or will core data figure that out for me, and migrate to the median version first, and then to the current version when a user loads an app version with the original data structure?

Thanks!

Answer 1

Depending on your model changes, you may use "The Default Migration Process". When calling addPersistentStoreWithType:configuration:URL:options:error: , use the flag NSMigratePersistentStoresAutomaticallyOption .

Also, I'm sure that unfortunately you need one mapping model for each possible migration. If you have 3 models, you'll have to implement: 1 to 3 and 2 to 3. Core Data is not smart enough to do the intermediate steps automatically :(

Answer 2

I guess the answer to whether or not you need to create another mapping model is... it depends. See Apple's docs here (specifically comments on lightweight migration): https://developer.apple.com/library/ios/documentation/cocoa/Conceptual/CoreDataVersioning/Articles/vmMigrationProcess.html#//apple_ref/doc/uid/TP40004399-CH6-SW1