**May be I am late here but @smokesnes, answer will not work for `many-to-many` relationship with same entity in Entity Framework Core version >= 2.1**
But his answer will work fine for the many to many relationship with different entity.
**Here is the way of configuring `many-to-many` relationship with same entity in EF Core >= 2.1 **
public class UserContacts
{
public int UserId { get; set; }
public virtual User User { get; set; }
public int ContactId { get; set; }
public virtual User Contact { get; set; }
}
public class User : DomainModel
{
public List<UserContacts> UserContacts { get; set; }
public List<UserContacts> ContactUsers { get; set; }
}
**Then in the model builder:**
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
base.OnModelCreating(modelBuilder);
modelBuilder.Entity<UserContacts>().HasKey(r => new {r.UserId, r.ContactId});
modelBuilder.Entity<UserContacts>()
.HasOne(pt => pt.User)
.WithMany(p => p.UserContacts)
.HasForeignKey(pt => pt.UserId).OnDelete(DeleteBehavior.Restrict);;
modelBuilder.Entity<UserContacts>()
.HasOne(pt => pt.Contact)
.WithMany(t => t.ContactUsers)
.HasForeignKey(pt => pt.ContactId).OnDelete(DeleteBehavior.Restrict);;
}
> So why does this happen? Is there anything I did wrong to map this
> many-to-many relationship?
No, you didn't do anything wrong. [It's just not supported][1]. Current status [here][2].
> Many-to-many relationships without an entity class to represent the
> join table are not yet supported. However, you can represent a
> many-to-many relationship by including an entity class for the join
> table and mapping two separate one-to-many relationships.
With EF-Core you should create the entity for the mapping table. Such as `UserContacts`. A complete example in the [docs][1], as mentioned in the comments. I haven't actually tested the code below, but it should look something like this:
public class UserContacts
{
public int UserId { get; set; }
public virtual User User { get; set; }
public int ContactId { get; set; } // In lack of better name.
public virtual User Contact { get; set; }
}
public class User : DomainModel
{
public List<UserContacts> Contacts { get; set; }
}
And your `modelBuilder`.
modelBuilder.Entity<UserContacts>()
.HasOne(pt => pt.Contact)
.WithMany(p => p.Contacts)
.HasForeignKey(pt => pt.ContactId);
modelBuilder.Entity<UserContacts>()
.HasOne(pt => pt.User)
.WithMany(t => t.Contacts)
.HasForeignKey(pt => pt.UserId);
[1]: https://docs.efproject.net/en/latest/modeling/relationships.html#many-to-many
[2]: https://github.com/aspnet/EntityFramework/issues/1368
