job.rb

class Job < ActiveRecord::Base

  STATUS_READY, STATUS_IN_PROGRESS, STATUS_DONE = 1, 2, 3

  def self.fetch(pid)
    update_all(
      { :status => STATUS_IN_PROGRESS,
           :pid => pid },
      { :status => STATUS_READY },
      { :limit  => 1,
        :order  => 'created_at asc' })
    find_by_status_and_pid STATUS_IN_PROGRESS, pid
  end

end