Why frozen test fixtures are a problem on large projects and how to avoid them

Author here, thanks for posting. :)

Example 1 bothers me. It says

> This test has just made it impossible to introduce another active project without breaking it, even if the scope was not actually broken. Add a new variant of an active project for an unrelated test and now you have to also update this test.

And then goes on to test that the known active projects are indeed included in what the call to Project.active returns.

However, that doesn't test that "active scope returns active projects". Rather, it tests that

- active scope returns _at least some of the_ active projects, and

And it does not test that

- active scope returns _all_ of the active projects

- active scope does not return non-active projects

Which, admittedly, is only different because the original statement is ambiguous. But the difference is that the test will pass if it returns non-active projects, too; which probably is not the expected behavior.

I prefer to set things up so that my test fixtures (test data) are created as close to the test as possible, and then test it in the way the article is saying is wrong (in some cases)... ie, test that the call to Project.active returns _only_ those projects that should be active.

Another option would be to have 3 different tests that test all those things, but the second one (_all_ of the active projects) is going to fail if the text fixture changes to include more active projects.

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.

Strongly agree with that. Its slower, but I will always prefer building actual database records as either part of the test or in the test context rather than relying on some predefined fixtures. That makes the test behaviour clearer, and it means you don't have a bunch of unrelated tests failing because someone changed a fixture to accommodate a new test.

Your test fixtures are introducing tighter coupling between the tests, than the code they are testing! In this scenario (a mock DB with data that a test relies on, which is incompatible with a new test you want to add), duplicating the fixture is correct. Different tests with incompatible requirements on state should use different mock data. In this specific case, however, it's also true that one of them can be modified to make the tests compatible. If you do red-green-refactor then you can duplicate first, and then coalesce by changing the first test.

"assert_equal names, names.sort" is a wrong answer. It would accept an empty collection.