rspec RSpec Mocks Mocking by setting a message expectation with expect

Example

The following example uses expect and receive to mock an Order's call to a CreditCardService, so that the test passes only if the call is made without having to actually make it.

class Order
  def cancel
     CreditCardService.instance.refund transaction_id
  end
end

describe Order do
  describe '#cancel' do
    it "refunds the money" do
      order = Order.new
      order.transaction_id = "transaction_id"
      expect(CreditCardService.instance).to receive(:refund).with("transaction_id")
      order.cancel
    end
  end
end

In this example the mock is on the return value of CreditCardService.instance, which is presumably a singleton.

with is optional; without it, any call to refund would satisfy the expectation. A return value could be given with and_return; in this example it is not used, so the call returns nil.