为什么不添加相关对象？ （实体框架）

Question

我有以下代码：

using (var context = new EF6RecipesContext())
{
   // add an artist with two albums
   var artist = new Artist { FirstName = "Alan", LastName = "Jackson" };
   var album1 = new Album { AlbumName = "Drive" };
   var album2 = new Album { AlbumName = "Live at Texas Stadium" };
   artist.Albums.Add(album1);
   artist.Albums.Add(album2);
   context.Artists.Add(artist);
   // add an album for two artists
   var artist1 = new Artist { FirstName = "Tobby", LastName = "Keith" };
   var artist2 = new Artist { FirstName = "Merle", LastName = "Haggard" };
   var album = new Album { AlbumName = "Honkytonk University" };
   artist1.Albums.Add(album);
   artist2.Albums.Add(album);
   context.Albums.Add(album);
   context.SaveChanges();
}
using (var context = new EF6RecipesContext())
{
   Console.WriteLine("Artists and their albums...");
   var artists = context.Artists;
   foreach (var artist in artists)
   {
      Console.WriteLine("{0} {1}", artist.FirstName, artist.LastName);
      foreach (var album in artist.Albums)
      {
         Console.WriteLine("\t{0}", album.AlbumName);
      }
   }
   Console.WriteLine("\nAlbums and their artists...");
   var albums = context.Albums;
   foreach (var album in albums)
   {
      Console.WriteLine("{0}", album.AlbumName);
      foreach (var artist in album.Artists)
      {
         Console.WriteLine("\t{0} {1}", artist.FirstName, artist.LastName);
      }
   }
}

我得到了我的程序的以下结果（请参见截图）：

在书中有不同的结果。

作者解释：

  

接下来，我们创建并初始化Artist实体类型的几个实例和Album实体类型的＆gt;实例。因为这两位艺术家合作了这张专辑，所以我们将这张专辑添加到两位艺术家的专辑导航属性（类型为＆gt; EntityCollection）。将相册添加到数据库上下文会导致艺术家获得   也加入了。

如果我们添加了专辑（“Honkytonk大学”），则应自动添加2个相关艺术家（“Tobby”和“Merle”）。但它们不会被添加（我查看了数据库）。为什么呢？