How to implement certain relation between two entities?

Question

I'm writing an app in asp.net MVC, and want to make models for my database using entity framework.

I want to make two entity models, song and playlist.

One playlist can, naturally, contain many songs, I suppose in that case I want to include

public List<Song> Songs { get; set; }

But, one song can be in more playlists, so how should I write song entity? Also

public List<Playlist> Playlists { get; set; }

?

In my app I want user to be able to add song to specific playlist. Only thing that comes into my mind is a little drop down menu next to a song that has listed all playlists which don't contain specific song, so I suppose I need song entity to have some kind of reference to all playlists which contain it.

Answer 1

This article on how to configure a Many-To-Many relationship should have some answers for you.

You will need to modify the model during ModelCreation something like this:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{

   modelBuilder.Entity<Song>()
               .HasMany<PlayList>(s => s.Playlists)
               .WithMany(p => p.Songs)
               .Map(cs =>
                        {
                            cs.MapLeftKey("PlayListID");
                            cs.MapRightKey("SongId");
                            cs.ToTable("PlayListSong");
                        });

}

You could also create this by creating your own class to sit in between:

public class PlayListSong
{
    public int PlayListID { get; set; }
    public int SongID { get; set; }
}

In database terms, this is known as an "Intersection Identity" table.