Mystery Guests in Tests

You’re having trouble understanding the behavior a test is verifying.

This testing anti-pattern is called Obscure Test. A common cause of Obscure Test is Mystery Guest:

The test reader is not able to see the cause and effect between fixture and verification logic because part of it is done outside the Test Method.

The impact is:

Identify the Mystery Guest

In the test suite of web applications, the Mystery Guest might be one of these smells:

Some smells have direct solutions:

Other smells require more thought.

Replace shared fixtures with fresh fixtures

Are your tests using shared fixtures like this?

dan:
  name: Dan
  role: developer
  location: San Francisco

phil:
  name: Phil
  role: designer
  location: Boston

Is it important in a specific test that the fixtures used are role-based (developer) or location-based (San Francisco)? If the former is true, we should rename the fixtures in order to reveal the intention: users(:developer) and users(:designer).

What we call “creating objects with factories”, others call creating Fresh Fixtures built using Inline Setup.

Each test method creates a test fixture for its own private use.

This meets our goal for the test reader to better see the cause and effect between fixture and verification logic.

Set up only relevant information

Irrelevant Information is another cause of Obscure Test. Again, Factory Girl is helpful. Consider this setup:

context "user account exists with a matching Facebook uid" do
  setup do
    @uid  = 1234567
    @user = create(:user, fb_user_id: @uid)
  end
  ...
end

We explicitly specify only the attribute on user that matters for this test. We use Factory Girl to create only a user with valid data, and we name a @uid variable to express intent for the otherwise Magic Number 1234567.

External resources

Sometimes we need to assert the contents of a file are what we expect. For example, say we’ve generated the file by dumping some XML from a web service which we’ll then stub out during test runs, using the file as a proxy.

context "the job XML from the web service" do
  setup { @xml_job = IO.read("test/fixtures/jobs/1.xml") }

  should "include the recruiter"s email" do
    recruiter_xml = "<recruiter>recruiter@example.com</recruiter>"
    assert_contains @xml_job, recruiter_xml
  end
end

In this case, test/fixtures/jobs/1.xml is called a Prebuilt Fixture.

To truly avoid a Mystery Guest here, the XML could be inline or have the recruiter’s email injected somehow.

Conclusion

Mystery Guest is memory trick to think about how code will be read and understood by humans. Can the intended behavior of a subset of the system be understood at a glance? Or is there a Mystery Guest clouding our understanding?