Question

Suppose I am doing matchmaking of users and games.
I have models containing users and games.

class Game < ActiveRecord::Base
  has_and_belongs_to_many :users 

class User < ActiveRecord::Base
  has_and_belongs_to_many :games

Games can have many users, users can be playing many games. Because of HASBM I have a table called games_users too.

I want to search and find games that are waiting for players, which do not also contain the username of the player (i.e. I don’t want to add the same player to a game twice…)

I want something like this:

@game = Game.find_by_status(Status::WAITING_USERS, :condition => “game.users.doesnt_contain(‘username=player’)

But I’m not sure how to do it?

Update:

Using jdl’s solution, I got the code to run, but get items that I tried to exclude returned in the results. Here’s my test code:

logger.debug "Excluding user: #{@user.id}"
games = Game.excluding_user(@user)
if (games != nil && games.count > 0)
  @game = Game.find(games[0].id)
  games[0].users.each {
   |u|
   logger.debug "returned game user: #{u.id}"
  }
end

(the above code also begs 2 questions…. – how do I get a result of just one game instead of an array, and how to I get a non-readonly version of it; that’s why I do the second Game.find…)

And here’s the output in the log:

Excluding user: 2
  Game Load (0.3ms)   SELECT `games`.* FROM `games` left outer join games_users gu on gu.game_id = games.id WHERE (gu.game_id is null or gu.user_id != 2) 
  Game Columns (1.0ms)   SHOW FIELDS FROM `games`
  SQL (0.2ms)   SELECT count(*) AS count_all FROM `games` left outer join games_users gu on gu.game_id = games.id WHERE (gu.game_id is null or gu.user_id != 2) 
  Game Load (0.1ms)   SELECT * FROM `games` WHERE (`games`.`id` = 3) 
  games_users Columns (6.8ms)   SHOW FIELDS FROM `games_users`
  User Load (0.9ms)   SELECT * FROM `users` INNER JOIN `games_users` ON `users`.id = `games_users`.user_id WHERE (`games_users`.game_id = 3 ) 
returned game user: 1
returned game user: 2

Answer 1

Named scopes are your friend here.

For example:

class Game < ActiveRecord::Base
  has_and_belongs_to_many :users

  named_scope :for_status, lambda {|s| {:conditions => {:status => s}}}
  named_scope :excluding_user, lambda {|u| {:conditions => ["gu.game_id is null or gu.game_id not in (select game_id from games_users where user_id = ?) ", u.id], :joins => "left outer join games_users gu on gu.game_id = games.id", :group => "games.id" }}
end

This will let you do things like the following:

user = User.first  # Or whoever.
games_in_progress = Game.for_status("playing")
games_in_progress_for_others = Game.excluding_user(user).for_status("playing")
# etc...

Also, since you say that you’re new to Rails, you might not realize that these named scopes will also work when you’re traversing associations. For example:

user = User.first
users_games_in_waiting = user.games.for_status("waiting")