Recording interactions on real methods

Okay, so imagine this situation: you have a unit test that, among other things, calls some API that doesn't return a meaningful value and you want to make sure that you've covered it. Since your test can't observe the result of the call, you want to make sure it happened

def roll_dice(dice)
  @mouth.blow!(dice)
  dice.roll
end

If a developer were simultaneously very eager to achieve an extreme amount of code coverage and also superstitious that actually blowing on the dice is very important, they could look at that @mouth.blow! invocation and ask "how can I verify that call happened while still calling through to its real implementation?"

Well, the answer if you're using Mocktail is, "sorry, you can't."

The primary reason this isn't supported is because Mocktail exists to facilitate test-driven development of code that's fully isolated from its dependencies in order to design well-considered interactions, and calling through to a dependency's actual implementation naturally violates that isolation. Additionally, for any reason a tester might wish to call through to the actual implementation other than superstition, it stands to reason that at most one of the following is possible:

1. An integrated test that doesn't use mocks could indirectly observe a dependency's side effect as evidence that a call occurred and therefore a mocking library isn't required to proxy it, or
2. It's not possible for the side effect of the call to be observed by either the test or its subject and therefore it's safe to achieve proper isolation by replacing the real implementation with a fake one

But both conditions can't be true. As a result, the urge to add a test assertion that a call occurred in a particular way may represent an unnecessary over-specification.

That said, if you're unswayed and still set on having this functionality, you can find it in the rr gem's mock.proxy API.

If you've heard enough, you can go back and consider non-TDD use cases for Mocktail.

Or if you're finally ready to walk the golden path, you can revisit Mocktail as a TDD tool.