Entity Framework : many to many relationship , Insert and update

Question

I have a news entity and I get the news based on their NewsID. then I defined a new entity , a Group and I want to get the news based on their Group ID. I tried to handle this (many to many) relationships using this article by using code first approach.

so in my context I added :

 public class Groupnews : DbContext
    {

        public DbSet<Group> Group { get; set; }
        public DbSet<News> News { get; set; }

        public Groupnews()
            : base("MyDb")
        {
        }

        public int NewsID { get; set; }
    }

this.HasMany(t => t.News)
    .WithMany(t => t.Groups)
    .Map(m =>
        {
            m.ToTable("GroupNews");
            m.MapLeftKey("GroupID");
            m.MapRightKey("NewsID");
        });

now I can get the news based on Their GroupID using this approach. but the problem is in insertign new News and updating.For that I need to save NewsID and GroupId in GroupNews table. for doing this . in News model i defined :

    public virtual ICollection<Group> RelatedGroups { get; set; }

    public News()
    {
        RelatedGroups = new List<Group>();
    }

and the same for group :

    public virtual ICollection<News> RelatedNews { get; set; }
    public Group()
    {
        RelatedNews = new List<News>();
     }

In my news controller I add :

            Group group = new Group();
            group.RelatedNews.Add(news);

but nothing is updated and the NewsID is not adding to my GroupNews table .

Answer 1

You should not define GroupNews separately. Meaning, you should not have a GroupNews class defined in your project. You have to do CRUD operations using independent associations (a list of News in Group class and a list of Group in your News class). Here's what your classes should look like:

public class Group
{
    ...
    public Group()
    {
         this.News = new List<News>();
    }

    public virtual ICollection<News> News {get;set;}
}

public class News
{
    ...
    public News()
    {
         this.Group = new List<Group>();
    }
    public virtual ICollection<Group> Groups {get;set;}
}

public class MyContext : DbContext
{
    public DbSet<Group> Groups { get; set; }
    public DbSet<News> News { get; set; }
}

Then you can use myGroup.News.Add(myNewsItem) or myNews.Groups.Add(myGroup) . Entity Framework will handle the insertion automatically. Notice you should use virtual keyword if you want to enable lazy loading for your associations.