Expect the creation of a new object

Question

I need to write an expectation that a new object will be created in a payment system. Code included from order.rb and order_spec.rb (Order class is here):

#order.rb 

def confirm_order(method_of_payment)
  if credit_card_but_products_out_stock?(method_of_payment)
    raise "Cannot make credit card payment now, as some products are out of stock" 
  end

  order_total
  Payment.new(method_of_payment, self.total)
end

#order_spec.rb

it 'it creates a new payment object if method_of_payment is valid and order.total is > 0' do
  order.add_product(product, 3)
  order.confirm_order(:credit_card)
  #Expect that a new payment object is created.
end

I want to understand how I can write the appropriate spec to test that the new Payment object is created. I found this article from Semaphore CI useful, but am not sure about a solution. I am pretty sure I should be creating a test double of some sort, and then maybe a method stub to allow(order).to receive(:confirm_order).and_return(#new_object??).

Answer 1

If you're unit testing the Order class, then you shouldn't be mocking or stubbing the Order class. You could set an expectation on the Payment class to return a mock that you can then further assert as the return value of confirm_order method.

let(:payment) { double :payment }

it "returns a payment" do
  expect(Payment).to receive(:new).with(:credit_card).and_return(payment)

  order.add_product(product, 3)
  expect(order.confirm_order(:credit_card)).to eq(payment)
end

This way, you are mocking the dependencies of your Order class, without altering the Order class, which is much safer as mocking method on Order could easily create false-positive test results.