Persistent store migration failed missing source managed object model

I'm trying to expand my Core Data. So I added a new attribute to my entity and tried using the Automatic Lightweight Migration. But when I'm starting the programm the error Persistent store migration failed missing source managed object model pops up.

Anyone knows what goes wrong?

The relevant part of my AppDelegate.c (in fact I only added NSDictionary *options):

- (NSPersistentStoreCoordinator *) persistentStoreCoordinator {


    if (persistentStoreCoordinator) return persistentStoreCoordinator;

    NSManagedObjectModel *mom = [self managedObjectModel];
    if (!mom) {
        NSAssert(NO, @"Managed object model is nil");
        NSLog(@"%@:%@ No model to generate a store from", [self class], _cmd);
        return nil;
    }  

    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSString *applicationSupportDirectory = [self applicationSupportDirectory];
    NSError *error = nil;

    if ( ![fileManager fileExistsAtPath:applicationSupportDirectory isDirectory:NULL] ) {
            if (![fileManager createDirectoryAtPath:applicationSupportDirectory withIntermediateDirectories:NO attributes:nil error:&error]) 开发者_如何学Go{
            NSAssert(NO, ([NSString stringWithFormat:@"Failed to create App Support directory %@ : %@", applicationSupportDirectory,error]));
            NSLog(@"Error creating application support directory at %@ : %@",applicationSupportDirectory,error);
            return nil;
            }  
    }  

    NSURL *url = [NSURL fileURLWithPath: [applicationSupportDirectory stringByAppendingPathComponent: @"stats.darx"]];
    persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel: mom];
    NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
                             [NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption,
                             [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption, nil];
    if (![persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType 
                                                configuration:nil 
                                                URL:url 
                                                options:options 
                                                error:&error]){
        [[NSApplication sharedApplication] presentError:error];
        [persistentStoreCoordinator release], persistentStoreCoordinator = nil;
        return nil;
    }   

    return persistentStoreCoordinator;
}  

You need to use a versioned Managed Object Model, containing both versions of your model. The automatic migration still needs to see both the existing and new versions of your model in order to work out what the differences are, and how to handle them.

The error you quote suggests that your app bundle now contains only your new model (the one you want to use), and not the old one (the one you're trying to migrate from). Go back into your version control system and retrieve the old model, then set up a versioned model containing the old model as v1 and the new one as v2.