Core Data Migration: How to delete the Core Data stack?

Question

My plan is to delete the old Core Data stack (the NSManagedObjectModel .momd file & the NSPersistentStore .sqlite file) because:

• I don't have experience with Core Data migrations.
• the new .xcdatamodel schema is completely different than the old one.
• I can safely delete the user's old data because it's all stored on our server and the new app downloads the latest data from our server anyway.

In this case, is complete deletion the best way to go about migration?

Answer 1

It is a perfectly valid thing to do if your app requires internet access anyway. Otherwise users may be left with an empty data set (you delete the old database when you find it's incompatible with the current model, but you cannot re-populate it without access to the server).

Technically, that's a trivial thing to do. When you set up the NSPersistentStoreCoordinator:

NSURL *storeURL = ...;
NSManagedObjectModel *managedObjectModel = ...;
NSError *error = nil;
_persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel: managedObjectModel];

// Check if we already have a persistent store
if ( [[NSFileManager defaultManager] fileExistsAtPath: [storeURL path]] ) {
    NSDictionary *existingPersistentStoreMetadata = [NSPersistentStoreCoordinator metadataForPersistentStoreOfType: NSSQLiteStoreType URL: storeURL error: &error];
    if ( !existingPersistentStoreMetadata ) {
        // Something *really* bad has happened to the persistent store
        [NSException raise: NSInternalInconsistencyException format: @"Failed to read metadata for persistent store %@: %@", storeURL, error];
    }

    if ( ![managedObjectModel isConfiguration: nil compatibleWithStoreMetadata: existingPersistentStoreMetadata] ) {
        if ( ![[NSFileManager defaultManager] removeItemAtURL: storeURL error: &error] )
            NSLog(@"*** Could not delete persistent store, %@", error);
    } // else the existing persistent store is compatible with the current model - nice!
} // else no database file yet

[_persistentStoreCoordinator addPersistentStoreWithType: NSSQLiteStoreType 
                                          configuration: nil 
                                                    URL: storeURL 
                                                options: nil 
                                                  error: &error];

Answer 2

If you create a blank Core Data application you find the necessary code in Apples comments in the Application Delegate:

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:@{NSMigratePersistentStoresAutomaticallyOption:@YES, NSInferMappingModelAutomaticallyOption:@YES} Lightweight migration will only work for a limited set of schema changes; consult "Core Data Model Versioning and Data Migration Programming Guide" for details.