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Mystery Guest

xUnit Test Patterns: Refactoring Test Code has an astounding amount of testing knowledge. Here's an example:

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:

  • Tests don't fulfill the role of Tests as Documentation.
  • You may have Erratic Tests which result don't pass during every test run or pass in the test environment but not in production.

Identify the Mystery Guest

In the test code of Rails applications, the Mystery Guest is often one of four smells:

  • a Rails fixture
  • a fixture file such as XML or JSON saved from an HTTP response
  • an instance variable defined at the top of a long context block, or nested up multiple context blocks
  • a variable without an Intention-Revealing Name

The last two smells have solutions:

  • pause and think before creating long context blocks
  • prefer flat over nested test files
  • pause and think about the intent of your variables, then name them that

The first two smells require more thought.

Replace fixtures with factories

An offensive Mystery Guest is Rails fixtures and Factory Girl is one defense.

Do you have fixture files like this?

  name: Dan
  role: developer
  location: San Francisco

  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).

Since the community has moved to factories, this testing behavior by Rails developers has been less common. 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)

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 ="test/fixtures/jobs/1.xml") }

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

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.

Writing "good" tests

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