EF 6.x: Can't seed tables with many-to-many relationship

Question

I'm currently experimenting with EF and I have following problem which I can't solve.

I have User and Role entities with many-to-many relationship. The problem appears when I'm trying to seed database with initial data. Two users and two roles(in teh code below) are seeded successfully. I can see entries in Roles and Users tables. But junction table has only one entry with user1 id and with role1 id . When I'm trying to get user with 2 roles from db, It has only one role - role1 . And I don't know why. Where is my mistake and how can I do this correctly? Here's my code:

Entity

public abstract class Entity
{
    public int Id { get; set; }
}

User

public class AppUser : Entity
{
    ...
    public virtual ICollection<AppRole> Roles { get; set; }

    public AppUser()
    {
        Roles = new SortedSet<AppRole>(new RoleComparer());
    }
}

Role

public class AppRole : Entity
{
    public RoleEnum Role { get; set; } 
    public ICollection<AppUser> Users { get; set; }
    public AppRole()
    {
        Users = new SortedSet<AppUser>(new UserComparer());
    }
}

FluentAPI

public class UserMap : EntityTypeConfiguration<AppUser>
{
    public UserMap()
    {
        ToTable("Users");
        ...
        #region Many-to-Many
        HasMany(usr => usr.Roles)
                .WithMany(r => r.Users)
                .Map(map =>
                {
                    map.ToTable("UsersAndRoles");
                    map.MapLeftKey("AppUserId");
                    map.MapRightKey("AppRoleId");
                });
        #endregion
    }
}

Seed code

public class DropCreateTestDbAlways : DropCreateDatabaseAlways<UnitTestContext>
{
    protected override void Seed(UnitTestContext context)
    {
        var role1 = new AppRole();
        var role2 = new AppRole() { Role = RoleEnum.Administrator };
        context.Roles.Add(role1);
        context.Roles.Add(role2);

        var user1 = new AppUser()
        {
            UserName = "RegularUser",
            Email = "regular@email.com",
            PasswordHash = "FGJSDBXNLSNLSDDSJSCLNCS",
            UserProfile = new AppUserProfile()
        };
        var user2 = new AppUser()
        {
            UserName = "AdminUser",
            Email = "admins@email.com",
            PasswordHash = "FGJSDBXNLSNLSDDSJSCLNCS",
            UserProfile = new AppUserProfile()
        };

        user1.Roles.Add(role1);
        user2.Roles.Add(role1);
        user2.Roles.Add(role2);

        context.Users.Add(user1);
        context.Users.Add(user2);
        base.Seed(context);
    }
}

Answer 1

Ok, I think I've found my problem. It seems that my Comparers where the cause. I replaced SortedSet<> with generic List, and everything started working as expected.

Here is the code of one of my comparers:

public class RoleComparer : IComparer<AppRole>
{
    public int Compare(AppRole x, AppRole y)
    {
        return x.Id.CompareTo(y.Id);
    }
}

I still can't quite understand why it was causing my problems, so if someone knows, please, tell me.