Comment by radanskoric | Hacker News Reader

Comment by radanskoric | Hacker News Reader

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Author here. Thanks for writing up your thoughts on this!

The "doesn't include non-active projects objections is easy", please check the Example 1 test again, there's a line for that:

``` refute_includes active_projects, projects(:inactive) ```

Hm, if you missed it, perhaps I should have emphasised this part more, maybe add a blank line before it ...

Regarding the fact that the test does not check that the scope returns "all" active projects, that's a bit more complex to address but let me let tell you how I'm thinking about it:

The point of tests is to validate expected behaviours and prevent regressions (i.e. breaking old behaviour when introducing new features). It is impossible for tests to do this 100%. E.g. even if you test that the scope returns all active projects present in the fixtures that doesn't guarantee that the scope always returns all active projects for any possible list of active projects. If you want 100% validation your only choice is to turn to formal proof methods but that's whole different topic.

You could always add more active project examples. When you write a test that is checking that "Active projects A,B and C" are returned that is the same test as if your fixtures contained ONLY active projects A,B and C and then you tested that all of them are returned. In either case it is up to you to make sure that the projects are representative.

So by rewriting the test to check: 1. These example projects are included. 2. These other example projects are excluded.

You can write a test that is equally powerful as if you restricted your fixtures just to those example projects and then made an absolute comparison. You're not loosing any testing power. Expect you're making the test easier to maintain.

Does that make sense? Let me know which part is still confusing and I'll try to rephrase the explanation.

I want to start by saying that I agree with what you're trying to accomplish here. And I agree with some of the ways you go about it. I'm trying to find the right words to covey what I mean here, but... the best I can come with is... what I'm saying here isn't "you're wrong because", it's "what you're doing seems to miss some situations; here's what I do that helps for those".

> The "doesn't include non-active projects objections is easy", please check the Example 1 test again, there's a line for that:

You're correct; I totally missed that.

> In either case it is up to you to make sure that the projects are representative.

That's fair, but that's also the point you're trying to address / make more robust by how you're trying to write tests (what the article is about). Specifically

- The article is about: How to make sure you're tests are robust against test fixtures changing

- That comment says: It's up to you to make sure your test fixtures don't change in a way that breaks your tests

> You can write a test that is equally powerful as if you restricted your fixtures just to those example projects and then made an absolute comparison. You're not loosing any testing power. Expect you're making the test easier to maintain.

By restricting your fixtures to just the projects (that are relevant to the test), you're making _the tests_ easier to maintain; not just the one test but the test harness as a whole. What I mean is that you're reducing "action at a distance". When you modify the data for your test, you don't need to worry about what other tests, somewhere else, might also be impacted.

Plus you do gain testing power, because you can test more things. For example, you can confirm it returns _every_ active project.

All that being said, what I'm talking about relies on creating the test data local to the tests. And doing that has a cost (time, generally). So there's a tradeoff there.

I think I'm getting what you mean and I almost completely agree with you, let me address one part, the only part where I don't agree:

> Plus you do gain testing power, because you can test more things. For example, you can confirm it returns _every_ active project.

Imagine this:

1. You start with some fixtures. You crafted the fixtures and you're happy that the fixtures are good for the test you're about to write.

2. You write a test where you assert the EXACT collection that is returned. This is, as you say, a test that "confirms the scope returns _every_ active project".

3. You now rewrite the test so that it checks that the collection includes ALL active projects and excludes all inactive projects.

Do you agree that nothing changed when you went from 2 to 3? As long as you don't change the fixtures, those 2 version of the test will behave exactly the same: if one passes so will the other and if one fails so will the other. As long as fixtures don't change they have exactly the same testing power.

If you agree on that, now imagine that you added another project to the fixtures. Has the testing power of the tests changed just because fixtures have been changed?

> If you agree on that, now imagine that you added another project to the fixtures. Has the testing power of the tests changed just because fixtures have been changed?

No, _but_ (and this is a big _but_) you're not testing the contract of the method, which (presumably) is to return all and only active projects.

Testing that it returns _some_ of the active methods is useful, but there are cases where it won't point out an issue. For example, image

- Over time, more tests are added "elsewhere" that use the same fixtures

- More active projects are added to the fixture to support those tests

- The implementation in the method is changed to be faster, and an off-by-one error is introduced; so the last project in the list isn't returned

In that ^ case, testing that _some_ of the active projects are returned will still return true; the bug won't be noticed.

Not directly related to the above, but I'll note that I would also split 2/3 into different tests.

- Make sure all projects returned are active

- Make sure projects returned includes all active projects

I think that's more of a style thing, but I _try_ to stick to each test testing one and only one thing. I don't always do that, but it's a rule of thumb for me.

I'm with you on the one assertion per test. I bundled two assertions into the same test here because my whole point was to have them effectively together describe a single test, just in a more maintainable manner.

Regarding the fact that I'm not fully testing the contract of the method, you're absolutely correct. But also, no example based test suite is fully doing that. As long as the test suite is example based it is always possible to find a counter-case where the contract is violated but the test suite misses it.

These counter-cases will be more contrived and less likely the better the test suite. So all of us at some point decide that we've done enough and that more contrived cases are so unlikely and the cost of mistake is so small that it's not worth it to put in the extra testing effort. Some people don't explicitly think about it but that decision is still made one way or another.

This is a long way of saying that I both agree with you but that also, in most cases, I would still take the tradeoff and go for more maintainable tests.

There's always a scenario where this can break though. What happens if someone introduces a test that confirms that marking `active1` as inactive works. Then it depends on the test order whether your initial test still passes.

It's required for tests to clean up after themselves. With Rails and fixtures this is handled by default: each test runs inside a transaction which is rolled back at the end of the test. That way each test starts with the same initial state.

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