How do I avoid a race condition in my Rails app?

Question

I have a really simple Rails application that allows users to register their attendance on a set of courses. The ActiveRecord models are as follows:

class Co         


        

Answer 1

Don't you just have to test if @run.full??

def create
   unless @user.valid? || @run.full?
      render :action => 'new'
   end

   # ...
end

Edit

What if you add a validation like:

class Attendance < ActiveRecord::Base
   validate :validates_scheduled_run

   def scheduled_run
      errors.add_to_base("Error message") if self.scheduled_run.full?
   end
end

It won't save the @attendance if the associated scheduled_run is full.

I haven't tested this code... but I believe it's ok.