EF代码中的模型n - n关系如何自动生成视图正常工作？

Question

I use EF Code First and have a problem in nn relationship, assume we have a singer that sing in some genres, so we need this models: Artist, Genre, and ArtistsGenres, I define Models as following:

This is My Artist Model:

public class Artist
{
    public long Id { get; set; }
    public string Name { get; set; }
    public ICollection<Genre> Genres { get; set; }
}

And My Genre Model:

public class Genre
{
    public long Id { get; set; }
    public string Title { get; set; }
    public ICollection<Artist> Artists { get; set; }
}

And my context class:

public class MusicDB : DbContex
{
    public DbSet<Artist> Artists { get; set; }
    public DbSet<Genre> Genres { get; set; }
    public DbSet<ArtistsGenres> ArtistsGenres { get; set; }

protected override void OnModelCreating(DbModelBuilder modelBuilder) {
modelBuilder.Entity<Artist>()
            .HasMany(a => a.Genres)
            .WithMany(g => g.Artists)
            .Map(model => {
                model.ToTable("ArtistsGenres");
                model.MapLeftKey("Artist_Id");
                model.MapRightKey("Genre_Id");
            });

        base.OnModelCreating(modelBuilder);
        }
}

But there is not any relationship between Artists and Genres when MVC generate views automatically.

For example, I need to change Genres of an Artist in edit view, in Create view I can set Genres for an Artist, or in Index View I want show genres for each Artist. But there isn't any generation for Genres in relation to Artist when MVC generate views automatically.

I know I can access both Genres and Artists from both side but I am interesting in MVC automatically generate views as we want: for ex: for each artist show related Genres.

How can I do this? Is my model correct? Is this true for any (n to n) relation that needs ICollection on both side? Or do I need some elements in overriding of OnModelCreating method in context class, for example something like this:

modelBuilder.Entity<Artist>()
    .HasMany(a => a.Genres)
    .WithMany(g => g.Artists);

Please help me, I don't know the exact implementation of NtoN relationship.

Answer 1

You haven't to create a separate Model for association between to models in a many-to-many relationship. Really the ArtistsGenres is not necessary. So, remove it, and you just have to change your modelBuilder to this one:

modelBuilder.Entity<Artist>()
    .HasMany(c => c.Genres)
    .WithMany(x => x.Artists)
    .Map(a => {
        a.ToTable("ArtistsGenres");
        a.MapLeftKey("ArtistId");
        a.MapRightKey("GenreId");
    });

It will use the ArtistsGenres table to map a many-to-many relationship between Artists table and Genres table automatically.

Note: When you define the ArtistsGenres model, EF will not look at it as a relationship, because you tell him that Hey EF, I have another model named ArtistsGenres ! Please manage it for me!!!

Your new entities and dbcontext will be these:

public class Artist {
    public long Id { get; set; }
    public string Name { get; set; }
    public ICollection<Genre> Genres { get; set; }
}

public class Genre {
    public long Id { get; set; }
    public string Title { get; set; }
    public ICollection<Artist> Artists { get; set; }
}

public class MusicDB : DbContex {

    public DbSet<Artist> Artists { get; set; }
    public DbSet<Genre> Genres { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder) {
    modelBuilder.Entity<Artist>()
        .HasMany(c => c.Genres)
        .WithMany(x => x.Artists)
        .Map(a => {
            a.ToTable("ArtistsGenres");
            a.MapLeftKey("ArtistId");
            a.MapRightKey("GenreId");
        });

}

Let me know if you have any questions or need clarifications on any part.

Answer 2

I would suggest you to go with the simpler approach of creating another model ArtistGenre and let EF figure out the relationship on its own. Create the table as below.

public class ArtistGenre
{
    public int Id;
    public int GenreId;
    public int ArtistId;

    public virtual Genre Genre;
    public virtual Artist Artist;
}

After that you will have another table added to the database by the above name with two foriegn key properties and one primary key.

Now, you can run the queries on this table. Say

var artist = myContext.ArtistGenre.where( g = g.GenreId == 1).ToList();

Now, artist wil hold all the artist under Genre with Id =1. You can do the vice-versa for Genres too in the similar way.

Hope it helps !!

Answer 3

The connection between Artist and genres is the ArtistsGenre.

So artist containt: ID, Name

And Genre contains: ID, Title

And ArtistsGenre contains: ID artist, ID genre

Answer 4

The problem is you are not explicitly loadin the Genres collection of the artist class and you do not allow EF to intercept that property access by not declaring it as virtual .

public class Artist
{
    public long Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Genre> Genres { get; set; }
}

Then when you need to access artist and the related genres you need to eager load them

var artist = db.Artists.Include(a => a.Genres)
      .Where(a => a.Name == "Foo").SingleOrDefault()

Making the Genres property virtual will allow EF to lazy load the collection if you didn't eager load it.