Minitest::Verify

Avoid false-positive tests by verifying they fail when key setup is removed.

This is a quick proof-of-concept minitest plugin, but it mostly works fine!

Install the gem and add to the application's Gemfile by executing:

$ bundle add minitest-verify

If bundler is not being used to manage dependencies, install the gem by executing:

$ gem install minitest-verify

This is a false-positive test. It always passes because post and comment are completely unrelated: there's no reason post.comments would ever include comment.

require "minitest/autorun"

class PostTest < Minitest::Test
  def test_comments_excludes_hidden_comments
    post = create(:post)
    comment = create(:comment, hidden: true)

    refute_includes post.comments, comment
  end
end

We can pull out the key setup and wrap it with verify_fails_without:

require "minitest/autorun"
require "minitest/verify"

class PostTest < Minitest::Test
  include Minitest::Verify

  def test_comments_excludes_hidden_comments
    post = create(:post)
    comment = create(:comment)

    verify_fails_without { comment.update!(hidden: true) }

    refute_includes post.comments, comment
  end
end

Now run the test with the --verify argument:

$ ruby post_test.rb --verify

This will cause the test to run twice. First it runs with the contents of the verify_fails_without block evaluated (normal run). Then it runs without the contents of the verify_fails_without block evaluated (verification run). If the test still passes without having evaluated the code inside the block, it's a false positive and you'll see a verification failure in your test output:

# Running:

F

Finished in 0.000380s, 2631.5783 runs/s, 5263.1565 assertions/s.

  1) Failure:
PostTest#test_comments_excludes_hidden_comments [post_test.rb:11]:
Expected at least one assertion to fail.

1 runs, 2 assertions, 1 failures, 0 errors, 0 skips