A Guide for Writing Maintainable Rails Tests

Do you ever feel like you spend most of your day repairing tests in your Rails app? If you have been building Rails apps for as long as we have, then you know the importance of a robust test suite. Working with a brittle and slow tests can really make the most basic tasks difficult. This is especially true for large Rails apps that have been around for a few years. The good news is that we can fix this with a few helpful tips I’ve picked up over the years that can keep your test suite running smoothly for the long-run.

First, let’s define what maintainable means. It can take on a lot meanings, but let’s break it down into three categories:

1. Tests should be reliable. When we run our tests over and over; whether it’s a single test or the entire test suite, we want them to consistently pass or even consistently fail. There’s nothing more tedious than tracking down inconsistencies. I’ve spent countless hours and sometimes days fixing tests that run fine in isolation, but fail when running the complete suite.

2. Tests should be easy to write. I confess, when I work with a Rails app that has slow or brittle tests, I skip writing new tests all together because I know it’s too difficult or too time consuming setting up new tests. So it’s critical that our test suite be straightforward enough to dive right in.

3. Tests should be easy to understand. When tests fail, we should be able to look at the test code and quickly understand why it’s failing and how to fix it. The last thing we want to do is waste time staring at code wondering “Why is this failing!”.

The Test Environment

Now that we have an understanding of what maintainable tests mean, let’s get started with a solid foundation - the test helpers. Here is what our test_helper.rb might look like:

require "minitest/autorun"
require "mocha/setup"
require 'simplecov'
require 'capybara/dsl'
require 'capybara/rails'

SimpleCov.start

module TestHelper
  include Capybara::DSL

  def setup
    DatabaseCleaner.strategy = :truncation
    DatabaseCleaner.clean_with(:truncation)
    DatabaseCleaner.start
  end

  def teardown
    DatabaseCleaner.clean
  end
end

Nice and simple! Let’s take a closer at the gems we’re using:

Minitest - I like to use MiniTest for writing tests because of its simplicity and it provides a complete set of testing facilities without the noise. Best of all it’s shipped with Ruby 1.9.

Mocha - When we absolutely need to mock or stub, I reach for Mocha since it’s a good lightweight option. I found Mocha to be the least fussy when it comes to stubbing.

SimpleCov is absolutely essential for measuring our code coverage. Our road to lean and mean tests start by knowing what has already been tested and what has yet to be tested.

DatabaseCleaner To avoid banging our head against bizarre inconsistencies with the test database, we are going to call on the help from DatbaseCleaner, which ensures that we have a clean database state between tests.

Capybara Capybara is an intuitive web browser simulation framework that allows us to test how a real user interacts with our web application. It also comes with a built-in DSL for describing user interactions. We’ll use Capybara extensively for our acceptance tests.

Achieving Testing Zen

Now that we have a nice setup for our suite, we’re ready to start adding tests. How we approach testing is important if we want to keep our test suite healthy. We want to write the minimal amount of code required to satisfy our test cases. The best way to do this is using the Top-Down approach. This brings us to our first tip.

Tip #1: Start at the top. Start with the acceptance tests and drive our implementation at the user interface level. This can be a great way to prototype our implementation, and starting at the top, we can cover a lot of code with little tests. Once we are satisfied with our acceptance tests, we can fill-in holes with unit tests for complete public interface coverage.

Tip #2: Use helper methods to keep your test DRY. To keep our tests nice and neat, extract common setup scenarios into helper methods. For example, if we know we have to log into the application to test our app, we can setup a handy login method:

def log_in_user(user)
  User.stubs(:authenticate).returns(user)
  visit "/login"

  within("form") do
    fill_in "login", with: user.login
    fill_in "password", with: "anything"
  end
  click_button "Log in"
end

Tip #3: Avoid Copy and Paste. Sometimes it’s so easy to copy and paste code from one test to another - especially our setup code. Before you copy and paste, ask yourself “Should I move this code to a helper method instead?”.

Tip #4: Don’t test what has already been tested. It may sound simple, but chances are you’ve written tests for code that has already been tested without knowing it. I come across something like this quite often:

describe User do
  it { should have_many(:orders)}
end

What exactly are we testing here? Are we just wanting to be extra sure that we’ve spelled the has_many :order association correctly in the User model? ActiveRecord associations are well-tested by the Rails test suite, so there is not need to have this in our test suite.

Tip #6: Avoid Excessive use of mocks, stubs, and expectations. Too many mocks and stubs can lead to unexpected results and expectations often serve no benefit at all and almost always lock you to the internal implementation of the thing you’re testing, thus making tests brittle. I have seen many projects use excessive mocks and often times it’s not testing anything. Prefer to use real data from the database. The downside of this of course, is that our tests may be slower.

Tip #7: Makes tests fast enough. We’re not focusing too much on speed. Don’t get me wrong, if your tests are slow, you won’t be encouraged to write them. So it’s important to keep speed in mind. But in the end, the most important thing we want is a reliable, understandable code, even if they’re not as fast as we’d like them to be.

Tip #8: Keep context and describe blocks flat. Avoid deeply nested describe contexts. For example:

describe "checking out" do

  describe "add items in shopping cart" do
    before do
      add_item_to_shopping_cart(product)
      click_link 'Checkout'
    end
    ...

    describe "pay with credit card" do
      ...
    end
  end
end

When go too deep in our describe blocks, it’s easy to lose sight of the steps taken in the previous describe blocks. To compound the problem, we can’t be sure of the database state either. Consider breaking describe blocks into separate tests or even separate files when deep nesting occurs. For our example above, we’ll create a brand new test file for paying with credit card:

describe "paying with credit card" do
  before do
    setup_shopping_cart_and_checkout
    fill_in_credit_card_fields
  end

  it 'checks for valid promotion codes' do
    fill_in "promo_code", with: 'does not exists'
    click_link 'Place Order'
    assert page.has_content?("Sorry, invalid promotion code")
  end
end

Tip #9: Only test public side effects. If a method changes the internal state, write your assertions based on the side effects available to the public. Keep test cases to external API - allows us to change internal implementation, while ensuring the consumers of your class still work.

And that is it! By following these tips, you’re on your way to achieving a happy and health test suite. If you found this article helpful, you should also checkout:

1. Practical Guide to User Testing
2. Rails Testing Pyramid
3. Minitest Quick Reference

Got any tips to share? Please leave them in the comments.