something.rb

class User < ActiveRecord::Base
  has_many :combo_instances
end

class ComboGame < ActiveRecord::Base
  has_many :combo_instances
  
  attr_reader :game_result

  validates :hits_required, presence: true
  validates :reward_points, presence: true
  
  def play(user)
    instance = combo_instances.where(user: user, game: self, status: :active).first
    instance ||= ComboInstance.create(user: user, game: self, status: :active)
    
    instance.play
  end
  
  def on_complete
    @game_result = reward_points
  end
end

class ComboInstance < ActiveRecord::Base
  belongs_to :combo_game
  belongs_to :user
  
  enum status: [:active, :complete]
  
  validates :hits, presence: true
  validates :status, presence: true
  
  after_save :check_complete
  
  def play
    hits += 1
    if hits == combo_game.hits_required
      status = :complete
      save!
    end
  end
  
  private
  
  def check_complete
    if status_changed? && status == :complete && status_was != :complete
      # the combo_game loaded here from the relationship seems to be a different instance than the one who called play
      combo_game.on_complete
    end
  end
end

u = User.new
game = ComboGame.create hits_required: 2, reward_points: 5
game.play(user)
game.game_result
# nil (expected)

game.play(user)
game.game_result
# nil (not expected should be 5 points)