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

asp.net-mvc ef-code-first entity-framework-6 many-to-many

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??

1
3
4/23/2015 6:37:40 PM

Accepted Answer

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; }
}
5
4/23/2015 7:35:54 PM

Popular Answer

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
}


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