class Vehicle < ActiveRecord::Base
  state_machine :initial => :parked do
    before_transition :parked => any - :parked, :do => :put_on_seatbelt
    after_transition any => :parked do |vehicle, transition|
      vehicle.seatbelt = '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
    ...
  end

  def benchmark
    ...
    yield
    ...
  end
end

Vehicle.with_state(:parked)                         # also plural #with_states
Vehicle.without_states(:first_gear, :second_gear)   # also singular #without_state