Robot Has No Heart

Here is a short 4 minute screencast in which I show you how the acts as state machine (AASM) gem fails in a concurrent environment, and also how to fix it.

(If embedding doesn’t work or the text is too small to read, you can grab a high resolution version direct from Vimeo)

It’s a pretty safe bet that you want to obtain a lock before all state transitions, so you can use a bit of method aliasing to do just that. This gives you much neater code than the quick fix I show in the screencast, just make sure you understand what it is doing!

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class ActiveRecord::Base
  def self.obtain_lock_before_transitions
    AASM::StateMachine[self].events.keys.each do |t|
      define_method("#{t}_with_lock!") do
        transaction do
          lock!
          send("#{t}_without_lock!")
        end
      end
      alias_method_chain "#{t}!", :lock
    end
  end
end

class Tractor
  # ...

  aasm_event :buy do
    transitions :to => :bought, :from => [:for_sale]
  end

  obtain_lock_before_transitions
end

This is a small taste of my DB is your friend training course, that helps you build solid rails applications by finding the sweet spot between stored procedures and treating your database as a hash. July through September I am running full day sessions in the US and UK. Chances are I’m coming to your city. Check it out at http://www.dbisyourfriend.com