ASP.NET, Entity Framework Code First many-to-many self referencing lists

Question

I'm trying to recreat a followers/followees system with Code First in ASP.NET. I can't find the right way to implement this (MVC pattern). There's my "User" class :

public class User
{
    public User()
    {
        this.Followers = new HashSet<User>();
        this.Follows = new HashSet<User>();
    }
    
    [Key]
    public int UserId { get; set; }
    public string Mail { get; set; }
    public string Password { get; set; }
    public string LastName { get; set; }
    public string FirstName { get; set; }
    public string Locality { get; set; }
    public string PhoneNumber { get; set; }
    public string Description { get; set; }
    public string ProfileImage { get; set; }

    public virtual ICollection<User> Followers { get; set; }
    public virtual ICollection<User> Follows { get; set; }
}

I have been able to create the database with the join table named "Subscriptions" with this dbcontext class :

public FakebookDBContext()
        : base("name=FakebookDBContext")
    {
    }

    public DbSet<User> Users { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {

        modelBuilder.Entity<User>()
                    .HasMany<User>(fer => fer.Follows)
                    .WithMany(fow => fow.Followers)
                    .Map(ff =>
                    {
                        ff.MapLeftKey("UserFollowerId");
                        ff.MapRightKey("UserFollowedId");
                        ff.ToTable("Subscription");
                    });

    }

While testing the Api related to my "User" class (generated automatically), i noticed two things :

While sending a POST request to my API with a User who follows an already existing User, it creates me two new User to the Database (the Subscription table is correclty filled tho)
While sending a PUT request to my API trying to change an already existing user Follows, the "Subscription" table remains unchanged.

Example for POST :

Input :

{
"$id" : 1,
    "Followers": [
        {
            "$id": "2",
            "Followers": [],
            "Follows": [{"$ref": "1"}],
            "UserId": 1,
            "Mail": null,
            "Password": null,
            "LastName": null,
            "FirstName": null,
            "Locality": null,
            "PhoneNumber": null,
            "Description": null,
            "ProfileImage": null
        }
    ],
    "Follows": [],
    "Mail": null,
    "Password": null,
    "LastName": null,
    "FirstName": null,
    "Locality": null,
    "PhoneNumber": null,
    "Description": null,
    "ProfileImage": null
}

Output :

{
"$id": "1",
"UserId": 24,
"Mail": null,
"Password": null,
"LastName": null,
"FirstName": null,
"Locality": null,
"PhoneNumber": null,
"Description": null,
"ProfileImage": null,
"Followers": [
    {
        "$id": "2",
        "UserId": 25,
        "Mail": null,
        "Password": null,
        "LastName": null,
        "FirstName": null,
        "Locality": null,
        "PhoneNumber": null,
        "Description": null,
        "ProfileImage": null,
        "Followers": [],
        "Follows": [
            {
                "$ref": "1"
            }
        ]
    }
],
"Follows": []

}

I concluded that i didn't achieve to link Users together correctly. It doesn't seem references to existing users are found and the framework(?) just creates new objects.

I would like to avoid to make a "Subscription" class and keep the User model that way (if possible). I'm quite new in IT so feel free to give any advice on how i should implement this better. Thank you !

It might be because of a missing "ForeignKey" attribute on UserId but i don't really know how to make it work. — Teodor Daue
Thanks for your answer, but as said in my question, i managed to create this join table called "Subscription" (see second code snippet) — Teodor Daue
You haven't posted any code from your save method. You need to ensure that you reload or attach existing User instances with their PK values. — Jeremy Lakeman

Sohaib Afzal Sohaib Afzal · Accepted Answer · 2020-10-30T04:10:30

I think the easiest way would be to create two other classes. One for followers and one for following. Each class would have one User and one ICollection<User>

I know you wanted to avoid making new classes but this is the simplest way to tell EF what you want to do.

ASP.NET, Entity Framework Code First many-to-many self referencing lists

1 Answers