There’s is a severe lack in documentation regarding heavyweight migration and apple docs is extremely lacklustre.

My old model used to have playlists and songs where they have a one to many relationship where one playlist contains many songs. The songs are ordered for each playlist thus was using an NSOrderedSet

In the new model, I want to remove this relationship, and go over song in a playlist and add the playlist UUID to the new “playlistId” attribute of each song.

Furthermore, since I want to retain the order of the songs were in in their respective playlists, each song will be assigned an index of 0 onwards so I can use a sort descriptor to sort the songs for a playlist.

I hope I am clear with my intention here.

class PlaylistSongMappingModel: NSEntityMigrationPolicy {
    override func createDestinationInstances(forSource sInstance: NSManagedObject, in mapping: NSEntityMapping, manager: NSMigrationManager) throws {
        
        if sInstance.entity.name == "Playlist" {
//Nothing is printed out...
            let songs = sInstance.mutableOrderedSetValue(forKeyPath: "songs")
            print(songs.array as? [Song])
        }
    }
}

extension Playlist {

    @nonobjc public class func fetchRequest() -> NSFetchRequest<Playlist> {
        return NSFetchRequest<Playlist>(entityName: "Playlist")
    }

    @NSManaged public var name: String?
    @NSManaged public var id: UUID?
}

extension Song {

    @nonobjc public class func fetchRequest() -> NSFetchRequest<Song> {
        return NSFetchRequest<Song>(entityName: "Song")
    }

    @NSManaged public var name: String?
    @NSManaged public var fileExtension: String?
    @NSManaged public var artist: String?
    @NSManaged public var index: Int64 //New attribute: The index at which the this song is in a playlist
    @NSManaged public var playlistId: UUID? //New attribute: The playlist the song is associated with.

    var wrappedName: String {
        name ?? "Unknown Song"
    }
    
    var wrappedFileExtension: String {
        fileExtension ?? "Unknown File Extension"
    }
    
    var wrappedArtist: String {
        artist ?? "Unknown Artist"
    }

var wrappedPlaylistId: UUID {
            playlistId ?? UUID()
        }
    }