Fluent NHibernate:如何在mapping类中映射“外键”列

public class Song
{
    public virtual int SongID{ get; private set; } //Primary Key
    public virtual int SongArtistID { get; set; } //Foreign Key mapping to 'Artist.ArtistID'
    public virtual string Title { get; set; }
}

public class Artist
{
    public virtual int ArtistID{ get; private set; } //Primary Key
    public virtual int ArtistName{ get; set; }
}

public class SongMapping : ClassMap<Song>
{
    SongMapping()
    {
        Id(c => c.SongID);//.GeneratedBy.HiLo("sermon"); is HiLo generator good?
        Map(c => c.SermonArtistID).Not.Nullable(); //How is this mapped to 'Artist.ArtistID'??
        Map(c => c.Title).Not.Nullable().Length(50);
    }
}