product_spec.rb

class Product
  attr_writer :price, :supplier
  
  def price
    @supplier ? @supplier.price_of(self) : @price
  end
end

class Supplier
  def price_of(product)
    1599
  end
end

describe Product do
  before do
    @product = Product.new
    @product.price = 1999
  end
   
  it "should use own price as its price" do
    @product.price.should == 1999
  end

  context "with supplier" do
    before do
      @supplier = Supplier.new
      @product.supplier = @supplier
    end
     
    it "should use supplier's price as its price" do
      @product.price.should == @supplier.price_of(@product)
    end
  end
end