How to prevent parallel accessing in RoR?

Question

There is controller

can_buy = nil
if game.spot_price == 0
  return {json: {error: 1, error_message: "You can do it only once"}, status: 400} if game.spots.where(user_id: user.id).length > 0
  can_buy = true
else
  can_buy = UserService.have_enough_money?(user, game.spot_price)
end
if can_buy
    ActiveRecord::Base.transaction do
      # purchase goes here
  end
end

And if user makes a few requests at the same moment and the spot_price is 0, he can buy few times, but he can do it only once. How to prevent it? PS: sorry for my english

Answer 1

Start a transaction earlier:

if game.spot_price.zero? && game.spots.where(user_id: user.id).length > 0
  return json: {error: 1, error_message: "You can do it only once"},
         status: 400
end
ActiveRecord::Base.transaction do
  if game.spot_price.zero? || UserService.have_enough_money?(user, game.spot_price)
    ...
  else
    raise ActiveRecord::Rollback, "Not permitted!"
  end
end