Q:

Implementation of "Automatic Lightweight Migration" for Core Data (iPhone)

I would like to make my app able to do an automatic lightweight migration when I add
new attributes to my core data model.
In the guide from Apple this is the only info on the subject I could find:

Automatic Lightweight Migration
To request automatic lightweight
migration, you set appropriate flags
in the options dictionary you pass in
addPersistentStoreWithType:configuration:URL:options:error:.
You need to set values corresponding
to both the
NSMigratePersistentStoresAutomaticallyOption
and the
NSInferMappingModelAutomaticallyOption
keys to YES:

NSError *error;
NSURL *storeURL = <#The URL of a persistent store#>;
NSPersistentStoreCoordinator *psc = <#The coordinator#>;
NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
    [NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption,
    [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption, nil];
 
if (![psc addPersistentStoreWithType:<#Store type#>
    configuration:<#Configuration or nil#> URL:storeURL
    options:options error:&error]) {
    // Handle the error.
}

My NSPersistentStoreCoordinator is initialized in this way:
- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
    
    if (persistentStoreCoordinator != nil) {
        return persistentStoreCoordinator;
    }
    
    NSURL *storeUrl = [NSURL fileURLWithPath: [[self applicationDocumentsDirectory] stringByAppendingPathComponent: @"FC.sqlite"]];
    
    NSError *error = nil;
    persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
    if (![persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeUrl options:nil error:&error]) {

        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }    
    
    return persistentStoreCoordinator;
}

I am having trouble seeing where and how I should add the Apple code to get the Automatic Lightweight Migration working?

A:

This is what I did to make Automatic Lightweight Migration (Source: http://brainwashinc.wordpress.com/2010/01/18/iphone-coredata-automatic-light-migration/)
1. Set the Persistent Store options for automatic migration in the app delegate.
Change your persistentStoreCoordinator creation to this (replace YOURDB):
- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {

  if (persistentStoreCoordinator != nil) {
    return persistentStoreCoordinator;
  }

  NSURL *storeUrl = [NSURL fileURLWithPath: [[self applicationDocumentsDirectory] stringByAppendingPathComponent: @"YOURDB.sqlite"]];

  // handle db upgrade
  NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
  [NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption,
  [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption, nil];

  NSError *error = nil;
  persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel: [self managedObjectModel]];
  if (![persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeUrl options:options error:&error]) {
    // Handle error
  }

  return persistentStoreCoordinator;
}

2. Version your Data Model and Edit the new file.
Select your xcdatamodel file
Design -> Data Model -> Add Model Version (expand your xcdatamodeld item)
Select the “2″ (or later) file, Design -> Data Model -> Set Current Version (edit this version)
3. Specify the momd resource in app delegate.
Change your managedObjectModel implementation to this (replace YOURDB)
- (NSManagedObjectModel *)managedObjectModel {

  if (managedObjectModel != nil) {
    return managedObjectModel;
  }

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

  return managedObjectModel;
}

A:

At first, the above solution didn't work for me. The returned managedObjectModel  was 0x0.
I think this is because I renamed the file names of the different model files.
If you follow the instructions above to the letter then it all works.
However if you do change model file names then you can select the "current" model file manually:
Lets say your original model file was MYMODEL.xcdatamodel
after doing the add model step above this turns into a directory MY.xcdatamodeld
and below it you have MYMODEL.xcdatamodel and MYMODEL 2.xcdatamodel
rename the new model file to whatever you want, for example lets say you removed the space to MYMODEL2.xcdatamodel and edit its content.
Now in the above code do
NSString *path = [mainBundle pathForResource:@"MYMODEL2" ofType:@"mom" inDirectory:@"MYMODEL.momd"];

