Rails 3 - Potential Race Condition? -

i have simple rails 3 application users can reserve 1 of finite number of homogeneous items particular day. i'm trying avoid race condition 2 people reserve last item available on particular day. model (simplified) follows:

class reservation < activerecord::base   belongs_to :user   attr_accessible :date   max_things_available = 20    validate :check_things_available    def check_things_available     unless things_available? errors[:base] << "no things available"   end    def things_available?     reservation.find_all_by_date(date).count < max_things_available    end            end 

the reservation being created in controller via current_user.reservations.build(params[:reservation])

it feels there better way this, can't quite put finger on is. on how prevent race condition appreciated.

not sure answers question, might point towards solution:

http://webcache.googleusercontent.com/search?q=cache:http://barelyenough.org/blog/2007/11/activerecord-race-conditions/

(the original site seems down that's link google cache)

the conclusion on page optimistic locking , row level locking not solutions race conditions on create, on update.

the author suggests reimplementing find_or_create db constraint.

another suggestion switching transaction isolation level 'serializable' ought work there's no information on how in rails.