问实体框架代码-优先-无法使用include检索嵌套对象

EN

public class Song
{
    [Key]
    public int Id { get; set; }
    public string Name { get; set; }
    [ForeignKey("OrchestraId")]
    public Orchestra Orchestra { get; set; }
    public int OrchestraId { get; set; }
    [ForeignKey("SingerId")]
    public Singer Singer { get; set; }
    public int SingerId { get; set; }
    public int Genre { get; set; }
    public string Year { get; set; }

    public Song()
    {
        this.Orchestra = new Orchestra();
        this.Singer = new Singer();
    }
}

public class Singer
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class Orchestra
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class POCContext : DbContext
{
    public DbSet<Singer> Singers { get; set; }
    public DbSet<Orchestra> Orchestras { get; set; }
    public DbSet<Song> Songs { get; set; }
}

    public Song GetSong(int songId)
    {
        Song song = new Song();

        song = _context.Songs.Include(s => s.Orchestra)
            .Include(s => s.Singer)
            .Single(s => s.Id == songId);
        return song;
    }