Question

Short:

How do you protect against race conditions on saving ActiveRecord objects in Ruby on Rails? Or is there at least a way to get an error if a race was detected?

Long:

Assume we have this Model

class Game < ActiveRecord::Base
  belongs_to :current_user, :class_name => "User", 
                  :foreign_key => "current_user_id"
  belongs_to :other_user, :class_name => "User", 
                  :foreign_key => "other_user_id
end

When creating games in the controller I want find the first game where :other_user_id is nil, set it to the user trying to create the game and then save the updated game.If no such game is found, create a new game with the asking user as current_user.

I can’t find anything in the documentation on ActiveRecord that shows how to protect against a race on saving the updated game i.e. user b could come in find the empty game and save to the database after user a has found the game but before they’ve saved to the database. If anything, what I’ve read on uniqueness validations seems like I need to fix this at the database level.

Thanks in advance!

Answer 1

You essentially want optimistic locking. This is enabled by having the lock_version column in game. So in a migration add something along the lines of:

 class AddLockedVersionToGames < ActiveRecord::Migration
   def changes
     add_column :games, :lock_version, :integer, :default => 0
   end
 end

And in the game model:

  def self.add_user_to_game(current_user)
    game = Game.find_by_other_user(nil)
    if game
      game.other_user = current_user
    else
      game = Game.new(current_user: current_user)
    end
    game.save
  rescue ActiveRecord::StaleObjectError
    game = Game.new(current_user: current_user)
    game.save
  end