ASP.NET MVC Many to Many relationship, using “My Own” table

Question

I am fairly new to using Code First approach with entity framework and I know that I you have a many to many relationship like the entities below, the EF will create the intermediary table automatically:

class Post {
  ...
  public virtual ICollection<Category> Categories {get; set;}
  ... 
}

class Category {
   ...
   public virtual ICollection<Post> Posts {get; set;}
   ...
}

However, if in the intermediary table I need to have extra data fields, one possible way (which I currently like, maybe because I am unaware of better ways) would be defining a new Entity of my own, like:

class Posts_Categories {
   public int Id {get; set;}
   public int CategoryId {get; set;}
   public int PostId {get; set;}
   public string Exrtafield1 {get; set;}
   public int ex extraField2 {get; set;}
   ...
   public virtual Post Post {get; set;}
   public virtual Category Category {get; set;}
}

Using this approach, EF does create my custom intermediary table, but it also creates another one of its own called "PostsCategories" which only contains a foreign key to Post_Id and another to Category_Id.

How do I make it not create that extra one and use the one I have defined? Is this a good way to manage Many to Many relationships with extra data fields??

Answer 1

you should use one to many relation like this :

public class Post
{
    public System.Int32 PostId { get; set; }

    [InverseProperty("Post")]
    public virtual ICollection<Posts_Category> PostCategories { get; set; }
}

public class Category
{
    public System.Int32 CategoryId { get; set; }

    [InverseProperty("Category")]
    public virtual ICollection<Posts_Category> PostCategories { get; set; }
}

public class Posts_Category
{
    public System.Int32 PostId { get; set; }

    public System.Int32 CategoryId { get; set; }

    [ForeignKey("PostId")]
    [InverseProperty("PostCategories")]
    public virtual Post Post { get; set; }

    [ForeignKey("CategoryId")]
    [InverseProperty("PostCategories")]
    public virtual Category Category { get; set; }
}

Answer 2

I needed to expand a bit on Iraj's answer to make it work. Another modification is that I'm including the default ApplicationUser as one of my tables.

So the relation is ApplicationUser 1-∞ IdeaVote ∞-1 Idea (i.e. there are users and ideas, users can vote on ideas, and each vote is represented with a connection between an ApplicationUser and an Idea.

public class Idea
{
    [Key]
    public int Id { get; set; }

    // This is an ordinary data field
    public string Text { get; set; }

    [InverseProperty("Idea")]
    public virtual ICollection<IdeaVote> Votes { get; set; }
}

public class IdeaVote
{
    [Key]
    public int Id { get; set; }

    public int IdeaId { get; set; }
    [ForeignKey("IdeaId")]
    [InverseProperty("Votes")]
    public virtual Idea Idea { get; set; }

    public string UserId { get; set; }
    [ForeignKey("UserId")]
    [InverseProperty("Votes")]
    public virtual ApplicationUser User { get; set; }
}

public class ApplicationUser : IdentityUser
{
    [InverseProperty("User")]
    public virtual ICollection<IdeaVote> Votes { get; set; }

    // Default stuff
}

Answer 3

It is normal for it to create that PostsCategories table for the relation between the two, and you are going to want that. If c is a Category, you'll be able to do things like c.Posts

Normally you would not create your own table manually for that. What kinds of data would you be keeping in the "extra" fields? I would probably move the fields to one of the other tables and drop that one. Most many to many relationship tables do not contain extra fields.