Question

Cracking my head while learning MVC through MVC MusicStore . Got this error for Model.Artist.Name for Details View page .

My Storecontroller Details method should be fine .

    public ActionResult Details(int id)
    {
        //returns and albums searched from the id 
        var albums = storeDB.Albums.Find(id);
        return View(albums);
    }

And this is how I output the view

    <li>Price : <%=Model.Price %></li>
    <li>Artist : <%=Model.Artist.Name%></li>

Works fine for the price , and it only show error for Model.Genre.Name and Artist.Name . I am suspecting the declaration for these properties in sampledata.cs caused this issue

    Artist = artists.Single(a => a.Name == "Aaron Copland & London Symphony Orchestra")

But my knowledge of this is too weak to make up anything . Please help .

Alright , the value is fetched from a class file looking like this

    protected override void Seed(MusicStoreEntities context)
    {
        var artists = new List<Artist>
        {
            new Artist { Name = "Aaron Copland & London Symphony Orchestra" },
            etcetc
        }
         new List<Album>
        {
            new Album { Title = "ABC", Artist= artists.Single(g => g.Name == "Test")}
        }
    }

Take notice on how the value are being assigned , I can access Model.Title fine (where Model as the short hand) , but Model.Artist.Name caused me that error .

Solved

Alright , got it to work by adding virtual keyword to the Artist and Genre declaration in Album class . But I am still not sure what happens and would like to have anyone care to shed some lights .

In my Album class , before it is solved it looks like this

public class Album
{
    public int AlbumId { get; set; }
    public int GenreId { get; set; }
    public int ArtistId { get; set; }
    public string Title { get; set; }
    public decimal Price { get; set; }
    public string AlbumArtUrl { get; set; }

    public Genre Genre { get; set; }
    public Artist Artist { get; set; }
}

The error caused by Genre and Artist is solved by adding Virtual keyword

    public virtual Artist Artist { get; set; }

Somehow , I still couldn’t justify what happens and wish to learn more . Anyone care to explain ?

It goes like this , Album.cs

namespace MvcMusicStore.Models
{
    public class Album
    {
        public int AlbumId { get; set; }
        public int GenreId { get; set; }
        public int ArtistId { get; set; }
        public string Title { get; set; }

        public virtual Genre Genre { get; set; }
        public virtual Artist Artist { get; set; }
    }
}

using EF MusicStore.cs

namespace MvcMusicStore.Models
{
    //represent entity framework , handle create ,read , update and del ops 
    public class MusicStoreEntities : DbContext
    {
        public DbSet<Album> Albums { get; set; }
        public DbSet<Genre> Genres { get; set; }
        public DbSet<Artist> Artists { get; set; }

    }
}

and implemented in StoreController.cs

    MusicStoreEntities storeDB = new MusicStoreEntities();

    public ActionResult Details(int id)
    {
        //returns and albums searched from the id 
        var albums = storeDB.Albums.Find(id);
        return View(albums);
    }

Finally the sample data

protected override void Seed(MusicStoreEntities context)
{
    var artists = new List<Artist>
    {
        new Artist { Name = "Aaron Copland & London Symphony Orchestra" },
        etcetc
    }
     new List<Album>
    {
        new Album { Title = "ABC", Artist= artists.Single(g => g.Name == "Test")}
    }
}

Answer 1

Alright , got it to work by adding virtual keyword to the Artist and Genre declaration in Album class . But I am still not sure what happens and would like to have anyone care to shed some lights .

See above .