class Vehicle
  include Mongoid::Document

  state_machine :initial => :parked do
    before_transition :parked => any - :parked, :do => :put_on_seatbelt
    after_transition any => :parked do |vehicle, transition|
      vehicle.seatbelt_on = 'off'
    end
    around_transition :benchmark

    event :ignite do
      transition :parked => :idling
    end

    state :first_gear, :second_gear do
      validates_presence_of :seatbelt_on
    end
  end

  def put_on_seatbelt
    self.seatbelt_on = 'on'
  end

  def benchmark
    #...
    yield
    #...
  end
end