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

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.

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

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.

Also, if you stumbled upon this post, like I did, after getting the "The model used to open the store is incompatible with the one used to create the store" error and you are just debugging using the simulator and wanting to completely replace the old model installed, you can just Reset the Simulator app or deleting your app from the simulator would probably work as well.

It didn't occur to me to try this until reading the posts here, at which point I realized that I had installed the app in the simulator and then subsequently changed the model, causing the aforementioned run-time error.

To follow up on Santthosh's answer, figured I'd post the code snippet right here instead. You need to create your managedObjectModel with initWithContentsOfURL: instead of mergedModelFromBundles: otherwise you'll get error:

Can't merge models with two different entities XXX and XXX

If your Model file is named "Model", here's how you create the managedObjectModel:

NSString *path = [[NSBundle mainBundle] pathForResource:@"Model" ofType:@"momd"];
NSURL *momURL = [NSURL fileURLWithPath:path];
managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:momURL]; 

Credit to this blog post.