I get the feeling a lot of RSpec users don’t know about the advantages of Rails 5.1 changes as part of the introduction of system tests. RSpec has had feature tests for a long time? What’s the big deal?


For context, RSpec has supported high level testing through feature tests for many years. Like Cucumber, feature tests are designed to exercise application functionality through the user interface. There are many merits to feature tests as a way to document core business logic and catch regressions. There are drawbacks as well, including the fact that they can be very expensive, i.e., slow, to maintain and execute. I'm going to ignore this for now.

I'll also altogether ignore interesting alternatives for UI and fullstack testing, like cypress.io. This post is aimed at folks working on Rails apps that have accumulated month or years worth of RSpec feature tests.

For a long time, Rails has resisted first class support for this kind of testing, so much of RSpec's integration with Rails is bolted on. It has mostly worked well, though there have been some gotchas which I'll get to.

Last year, that all changed when Rails released Rails 5.1 with some key changes to support the introduction of system tests. On the surface, Rails system tests have pretty much the same usage and goals as RSpec feature tests, including integration with Capybara, the Ruby interface to interacting with numerous webdrivers like headless Chrome, Gecko for Firefox, PhantomJS, etc. So basically everything we've had in RSpec for ages. RSpec developers collectively yawned.

The real win with Rails 5.1 is what's happening under the surface. The Rails team made key changes to internals, stuff RSpec alone couldn’t provide.

Why transactional tests are better

Awhile back, RSpec feature tests, through Capybara, enabled developers to test UI interactions enabled by JavaScript. With so much logic in our apps going to the frontend, this was a huge improvement over standard Rails testing support, which previously stopped well short of that.

Though this helped increase testing confidence (let’s ignore flaky JS tests for now), this approach also came with a catch: developers had to think about managing the database in their end-to-end JavaScript-enabled acceptance tests.

Backing up a step, RSpec tests are typically individually wrapped in database transactions. This makes rolling back DB changes that occur within each test really fast and easy. But in pre-Rails 5.1, RSpec feature tests with a JavaScript-enabled webdriver like Chrome, wrapping tests in transactions doesn’t work!

Previously, it was not possible for the test and server threads to share the same database connection for JavaScript-enabled drivers; any data created in a transaction in the test isn’t committed to the DB, so the Rails server doesn’t have access to the data! Missing data in the JavaScript acceptance tests is really confusing to lots of Rails developers, myself included. To this day, this “gotcha” has tripped me up on new projects.

The workaround, for years, has been to disable transaction mode for—the very feature that makes database-backed test faster and easier to rollback for successive tests—for JavaScript-enabled feature tests. To replace this, most RSpec-based Rails projects lean on another gem, DatabaseCleaner, plus some extra configuration, to switch modes just for JavaScript-enabled acceptance tests. The alternative modes are usually either truncate the whole DB or delete all the rows; both slower and sometime problematic when switching back-and-forth. All this instead of just having RSpec rollback transactions without us having to think about it while developing our tests.

Not to mention, having the Rails server run in a separate process makes it a lot harder to debug. If you like using a debugger like pry in your application code, good luck making it work with traditional RSpec acceptance tests.

Rails 5.1+ solves the database problem. Eileen Uchitelle on the Rails team made the changes necessary to run ensure test threads and the Rails server can run in the same process by sharing the database connection (pull request). This made it possible to wrap JavaScript-enabled acceptance tests in database transactions. To take advantage, RSpec users would need to upgrade to a recent version of Rails, re-enable transactional fixtures for all tests, and remove the DatabaseCleaner gem.

The result: faster rollback, no multiprocess confusion, no need to manage the database with DatabaseCleaner, debugging the server in process is possible, etc. A better solution all around.

In closing

To my RSpec friends: upgrade to Rails 5.1, drop the DatabaseCleaner gem, and set config.use_transactional_fixtures = true in the RSpec configuration. It should also be relatively straightforward to adopt system tests from existing feature tests, but either strategy will work with those changes. You’ll still need to fix those flaky scenarios yourself though.


This post has been edited to reflect Thomas Walpole's corrections in the comments.

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Photo by Paola Galimberti on Unsplash

Published on Dec 27, 2018