EF Core - Many to many relationship on a class

Question

User-Friend relationship

I find an answer

Entity Framework Core: many-to-many relationship with same entity and try like this.

Entitys:

public class User
{
    public int UserId { get; set; }

    public virtual ICollection<Friend> Friends { get; set; }
}

public class Friend
{
    public int MainUserId { get; set; }

    public User ManUser { get; set; }

    public int FriendUserId { get; set; }

    public User FriendUser { get; set; }
}

The fluent API:

modelBuilder.Entity<Friend>()
    .HasKey(f => new { f.MainUserId, f.FriendUserId });

modelBuilder.Entity<Friend>()
    .HasOne(f => f.ManUser)
    .WithMany(mu => mu.Friends)
    .HasForeignKey(f => f.MainUserId);

modelBuilder.Entity<Friend>()
    .HasOne(f => f.FriendUser)
    .WithMany(mu => mu.Friends)
    .HasForeignKey(f => f.FriendUserId);

When I Add-Migration, the error message is

Cannot create a relationship between 'User.Friends' and 'Friend.FriendUser', because there already is a relationship between 'User.Friends' and 'Friend.ManUser'. Navigation properties can only participate in a single relationship.

What should I do? Or I should create an Entity FriendEntity:User?

Answer 1

The problem is that you can't have one collection to support both one-to-many associations. Friend has two foreign keys that both need an inverse end in the entity they refer to. So add another collection as inverse end of MainUser:

public class User
{
    public int UserId { get; set; }
    public virtual ICollection<Friend> MainUserFriends { get; set; }
    public virtual ICollection<Friend> Friends { get; set; }
}

And the mapping:

modelBuilder.Entity<Friend>()
    .HasKey(f => new { f.MainUserId, f.FriendUserId });

modelBuilder.Entity<Friend>()
    .HasOne(f => f.MainUser)
    .WithMany(mu => mu.MainUserFriends)
    .HasForeignKey(f => f.MainUserId).OnDelete(DeleteBehavior.Restrict);

modelBuilder.Entity<Friend>()
    .HasOne(f => f.FriendUser)
    .WithMany(mu => mu.Friends)
    .HasForeignKey(f => f.FriendUserId);

One (or both) of the relationships should be without cascading delete to prevent multiple cascade paths.

Answer 2

It's not mandatory the second collection. You only need to left de .WithMany() empty like this:

modelBuilder.Entity<Friend>()
    .HasOne(f => f.MainUser)
    .WithMany()
    .HasForeignKey(f => f.MainUserId);

modelBuilder.Entity<Friend>()
    .HasOne(f => f.FriendUser)
    .WithMany()
    .HasForeignKey(f => f.FriendUserId);

look at this : https://github.com/aspnet/EntityFramework/issues/6052