What do I have to do to get Core Data to automatically migrate models?

This was incredibly helpful. The Apple documentation was -- as usual -- woefully incomplete. I recommend doing a clean build, as I ran into an error "Can't merge models with two different entities xxx" when I first ran after making these changes. The clean build fixed it up.

I've now found out that this is quite simple - once you know where to look.

In my AppDelegate I set-up the NSPersistentStoreCoordinator - and you need to add some options to this to tell it to handle auto-migrate:

NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:

[NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption,

[NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption, nil];

NSError *error;
_persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel: [self managedObjectModel]];

if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeUrl options:options error:&error]) {
    // Handle error
    NSLog(@"Problem with PersistentStoreCoordinator: %@",error);
}

Then you need to do a little trick in xCode:

1. Select your xcdatamodel file
2. Select the Design Menu at the top - then Data Model - then choose Add Model Version
3. Your xcdatamodel file will then get moved into a new directory with the same name as your xcdatamodel file but with the extension xcdatamodeld - there will be a second file in this directory with a 2 in the name. Select the new file and then Design->Data Model->Set Current Version (in Xcode 4 you do this)
4. If you have already made the changes that have caused your project to be incompatible - take these changes out of the original xcdatamodel file. If you have yet to make the changes - then just edit the 2.xcdatamodel file (the one you just made current version).
5. Now when you install this version onto a device that has the old model - it will automatically upgrade that model to the new model.

This seems great and as simple as I wanted - but I think you need to be careful during development as you change a model - otherwise you will have to create a new version for each change.

I think what I will do is that I will keep all of the changed files and then once I get ready to deploy my update I'll delete all the in-between files and just deploy with the oldest and latest models.

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UPDATE (15/07/2011):

Thanks to @rockstarberlin for pointing out there is updated documentation at apple:

Xcode 4: Setting a Managed Object Model’s Current Version

Update: 8/19/2013 better link:

https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/CoreDataVersioning/Articles/vmModelFormat.html