http://blog.guiltry.com/strategy-pattern-vs-state-pattern

order_with_strategy.rb

class Order < ActiveRecord::Base
  after_initialize :change_strategy
  
  def set_paid_strategy(paid_strategy)
    @paid_strategy = paid_strategy
  end

  def set_unpaid_strategy(unpaid_strategy)
    @unpaid_strategy = unpaid_strategy
  end
  
  def change_strategy
    if self.status == Order::PAID
      self.set_paid_strategy(PaidOrderPaidStrategy.new)
      self.set_unpaid_strategy(PaidOrderUnpaidStrategy.new)

      # Commented code below is an example how you can change your paid strategy
      # Pure State Pattern can't do it.
      # self.set_paid_strategy(PaidOrderPaidStrategyWithoutEmail.new) 
    elsif self.status == Order::UNPAID
      self.set_paid_strategy(UnpaidOrderPaidStrategy.new)
      self.set_unpaid_strategy(UnpaidOrderUnpaidStrategy.new)
    end
  end
  
  def paid!
    @paid_strategy.paid!
  end
  
  def unpaid!
    @unpaid_strategy.unpaid!
  end
end