Scope users remaining games based on a third table with Laravel

I try to do a scope to easy get the users not already played games, the table structures looks like:

users

• id
• username

games

• id

user_games

• id
• user_id
• game_id

Here is a sqlfiddle: http://sqlfiddle.com/#!9/fe9d19/3

If the user already have played game with ID 2, it should no longer count with it in games_left. So to just to be clear, the result I try to get is something like:

{
    "id" => 1,
    "username" => "test",
    "games_left" => 2
}

any help is much appreciated, thanks.

via Chebli Mohamed