gistfile1.rb

class Task

  after_save :nofity_for_next_tasks

  def notify!
    unless user_notified? # This is a new column
      send_actual_email_somehow(task, user)
      update_attribute(:user_notified, true)
    end
  end

private

  def notify_for_next_tasks
    candidate.nofity_for_next_tasks
  end

end

class Candidate

  def notify_for_next_tasks
    current_stage = find_current_stage_somehow
    current_stage.tasks.each do |task|
      task.notify!
    end
  end

end