http://blog.guiltry.com/ruby-on-rails-with-state-pattern-at-its-finest

order.rb

class Order < ActiveRecord::Base
  after_initialize :change_state
  
  def set_state(order_state)
    @state = order_state
  end
  
  def change_state
    # we keep this logic block, to choose what state logic the object use at initialization
    if self.status == Order::PAID
      self.set_state(OrderPaidState.new)
    elsif self.status == Order::UNPAID
      self.set_state(OrderUnpaidState.new)
    end
  end
  
  def paid!
    @state.paid!
  end
  
  def unpaid!
    @state.unpaid!
  end
end

order_state.rb

class OrderUnpaidState
  def unpaid!(order)
    # do nothing or raise an error
  end
  
  def paid!(order)
    # sent email, change status, ...
    order.set_state(OrderPaidState.new)
  end
end

class OrderPaidState
  def unpaid!(order)
    # sent email, change status, ...
    order.set_state(OrderUnpaidState.new)
  end
  
  def paid!(order)
    # do nothing or raise an error
  end
end