Random Musings of an Insane Mind

I ran into this while doing some development for SnapReplay, and, I knew that this would be a problem. Regrettably, the favorite solution is to delete the app or erase the virtual instance and start fresh, but, what causes this?

When your application first starts and you have a schema defined for SQLite, a database file is created and versioned. When you later make changes to the table schemas or add a new table, the version number is updated to allow migration. People that have downloaded your app are not going to want to uninstall the app and reinstall it.

So, we’ve made changes to our table schema, added or modified a table or whatever. Now, we need to go to our list of files, right click, New File… IOS Core Data, NSManagedObject subclass, hit next, select the Data Model for your project, select the entities. If you have only added a new table, select it, if you have modified tables, select them as well. Hit next, then, create the files.

Now, you have a few options. If your changes are slight, you can use the lightweight migration (in your App Delegate file):

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator
{
    if (__persistentStoreCoordinator != nil)
    {
        return __persistentStoreCoordinator;
    }
    
    NSError *error = nil;
    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"SnapReplay.sqlite"];
    
    NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption, [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption, nil];
    
    __persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];

For a more complete discussion regarding more complicated data migrations, you can refer to Apple’s Core Data Model Versioning and Data Migration guide.