Question

Every time I change the Core Data model for my app, it generates an unrecoverable error at the next startup: “The model used to open the store is incompatible with the one used to create the store”.

The only reliable way to avoid this I have found is to manually delete the app and let Xcode reinstall it, or use other techniques to manually blow away the Core Data store .sqlite store file. This is obviously not viable for shipping out to users.

Apple’s default App Delegate template for initializing the NSPersistentStoreCoordinator includes this comment:

 __persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
if (![__persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) {
    /*
     TODO: Replace this implementation with code to handle the error appropriately.

     ...

     If you encounter schema incompatibility errors during development, you can reduce their frequency by:
     * Simply deleting the existing store:
     [[NSFileManager defaultManager] removeItemAtURL:storeURL error:nil]

     * Performing automatic lightweight migration by passing the following dictionary as the options parameter: 
     [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption, [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption, nil];

     */

I have not been able to find any sample or instructions on how to “handle the error appropriately” though.

I would be happy to simply blow away the database in this case and let the app regenerate it from online data. This is what I do when I blow the database away manually. But how can I do that automatically in response to this error condition?

Are there any good samples explaining best practices?

Answer 1

Best way is to use lightweight migration. See Apple documentation: http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CoreDataVersioning/Articles/vmLightweightMigration.html.

When you need to make changes in model in new app version, you create new model. You do this in Xcode – just select Your current model and select from menu Editor/Add model version… Without this, automatic migration will not work.