Modeling that app

gistfile1.rb

class Player < ActiveRecord::Base
  has_many :initiated_challenges, foreign_key: 'player_one_id', class_name: 'Challenge'
  has_many :accepted_challenges, foreign_key: 'player_two_id', class_name: 'Challenge'
end

class Challenge < ActiveRecord::Base
  belongs_to :player_one, class_name: 'Player'
  belongs_to :player_two, class_name: 'Player'

  has_many :games
end

class Game < ActiveRecord::Base
  belongs_to :challenge
end