如何在Entity Framework Core中将项目添加到多对多关系表中？

Question

In my EF Core data model I have two table, the first table have an ICollection Navigation property to the second table and the second table also have an ICollection Navigation property to the first table.

Data Model

public class MyUser
{
    [Key]
    public string UserName { get; set; }
    public ICollection<Vote> Votes { get; set; }
    public virtual ICollection<PrivateUser> Rooms { get; set; }
}

public class PrivateUser
{
    [Key]
    public string RoomName { get; set; }
    public virtual ICollection<MyUser> Users { get; set; }
}

When I first add a MyUser only with name,

 appuser = new MyUser
 {
      UserName = Context.User.Identity.Name
 };
 _context.MyUser.Add(appuser);
 _context.SaveChanges();

it's adding the user to the table correctly but when I try add a PrivateUser with an existing MyUser or I try to update the existing MyUser it gives NullReferenceException from the below code.

PrivateUser roma = new PrivateUser() { RoomName = roomName};
try
{
    var user = new MyUser() { UserName = Context.User.Identity.Name };
    user.Rooms.Add(roma);
    _context.MyUsers.Attach(user).Property(u => u.Rooms).IsModified = true;
    roma.Users.Add(user);
    await _context.Rooms.AddAsync(roma);
    await _context.SaveChangesAsync();
}
catch(Exception ex)
{
    Console.WriteLine(ex.ToString());
}

I've also configured it in the ModelBuilder.

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
     base.OnModelCreating(modelBuilder);
     modelBuilder.Entity<MyUser>().HasMany(u => u.Rooms);
     modelBuilder.Entity<PrivateUser>().HasMany(c => c.Users);

}

I'm a beginner on EF Core, can somebody please explain me how I can save the fields in correct way when the both table referencing themselves.

Answer 1

After a long investigate I found there was no problem with my Entity Model the problem was because I solved that problem on my first Migration by using ModelBuilder

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder);
    modelBuilder.Entity<MyUser>().HasMany(u => u.Rooms);
   modelBuilder.Entity<PrivateUser>().HasMany(c => c.Users);
}

The problem was in my try block, see the correct code below.

try
{
    var user = new MyUser() { UserName = Context.User.Identity.Name };
    //here I was doing user.Rooms.Add(roma); where Romms is null
    roma.Rooms = new ICollection<PrivateUser>();
    //now we can add the room to Rooms
    user.Rooms.Add(roma);
    //and here was the below commented code from a Stackoverflow Answer
    //_context.MyUsers.Attach(user).Property(u => u.Rooms).IsModified = true;
    //we just have to use Update here;
    _context.MyUsers.Attach(user);
    _context.MyUsers.Update(user);
    roma.Users.Add(user);
    await _context.Rooms.AddAsync(roma);
    await _context.SaveChangesAsync();
}

Now everything fine as expected. Thanks...