decorators. see http://robots.thoughtbot.com/post/14825364877/evaluating-alternative-decorator-implementations-in comment by Clayton

cream_and_sugar.rb

require "money"

class Decorator < BasicObject
  def initialize(component)
    @component = component
  end

  def method_missing(name, *args, &block)
    @component.send(name, *args, &block)
  end

  def send(symbol, *args)
    __send__(symbol, *args)
  end

  def ==(other)
    @component == other
  end
end

class Cream < Decorator
  def cost
    @component.cost + ::Money.new(50, "USD")
  end
end

class Sugar < Decorator
  def cost
    @component.cost + ::Money.new(20, "USD")
  end
end

class Coffee
  def cost
    ::Money.new(200, "USD")
  end

  def origin
    "Columbia"
  end
end

cream_and_sugar_spec.rb

describe Coffee do
  its(:cost)   { should == 2 }
  its(:origin) { should == "Columbia" }
  it           { should be_an_instance_of(Coffee) }
end

describe "Coffee with Sugar" do
  let(:coffee) { Coffee.new }
  subject      { Sugar.new(coffee) }

  it              { (subject == coffee).should be_true }
  its(:cost)      { should == 2.2 }
  its(:origin)    { should == "Columbia" }
  its(:object_id) { should == coffee.object_id }
  its(:class)     { should == coffee.class }
  it              { should be_an_instance_of(Coffee) }
end

describe "Coffee with two Sugar" do
  let(:coffee) { Coffee.new }
  subject      { Sugar.new(Sugar.new(coffee)) }

  it              { (subject == coffee).should be_true }
  its(:cost)      { should == 2.4 }
  its(:origin)    { should == "Columbia" }
  its(:object_id) { should == coffee.object_id }
  its(:class)     { should == coffee.class }
  it              { should be_an_instance_of(Coffee) }
end

describe "Coffee with Sugar and Cream" do
  let(:coffee) { Coffee.new }
  subject      { Sugar.new(Cream.new(coffee)) }

  it              { (subject == coffee).should be_true }
  its(:cost)      { should == 2.7 }
  its(:origin)    { should == "Columbia" }
  its(:object_id) { should == coffee.object_id }
  its(:class)     { should == coffee.class }
  it              { should be_an_instance_of(Coffee) }
end