How do I specify "not in" in this lambda expression?

Question

I have a quick question becuase my brain won't work with me...
Where do I specify that I want the user_id's in 'Users' that are NOT in 'Groups' ?

db.Users.Join(db.Groups, a => a.user_id, b => b.user_id, (a, b) => new SelectListItem
{
  Value = a.user_id.ToString(),
  Text = a.surname + " " + a.lastname
});

Answer 1

The following should work (assuming that I understood your question correctly):

db.Users
.Where(x => !db.Groups.Any(y => y.user_id == x.user_id))
.Select(a => new SelectListItem
    {
        Value = a.user_id.ToString(),
        Text = a.surname + " " + a.lastname
    });