cypress-rails

This is a simple gem to make it easier to start writing browser tests with Cypress for your Rails apps, regardless of whether your app is server-side rendered HTML, completely client-side JavaScript, or something in-between.

Installation

tl;dr:

1. Install the npm package cypress
2. Install this gem cypress-rails
3. Run rake cypress:init

Installing Cypress itself

The first step is making sure Cypress is installed (that's up to you, this library doesn't install Cypress, it just provides a little Rails-specific glue).

If you're on newer versions of Rails and using webpacker for your front-end assets, then you're likely already using yarn to manage your JavaScript dependencies. If that's the case, you can add Cypress with:

$ yarn add --dev cypress

If you're not using yarn in conjunction with your Rails app, check out the Cypress docs on getting it installed. At the end of the day, this gem just needs the cypress binary to exist either in ./node_modules/.bin/cypress or on your PATH.

Installing the cypress-rails gem

Now, to install the cypress-rails gem, you'll want to add it to your development & test gem groups of your Gemfile, so that you have easy access to its rake tasks:

group :development, :test do
  gem "cypress-rails"
end

Once installed, you'll want to run:

$ rake cypress:init

This will override a few configurations in your cypress.json configuration file.

Usage

Develop tests interactively with cypress open

When writing tests with Cypress, you'll find the most pleasant experience (by way of a faster feedback loop and an interactive, easy-to-inspect test runner) using the cypress open command.

When using Rails, however, you'll also want your Rails test server to be running so that there's something for Cypress to interact with. cypress-rails provides a wrapper for running cypress open with a dedicated Rails test server.

So, by running:

$ rake cypress:open

Any JavaScript files added to cypress/integration will be identified by Cypress as tests. Simply click a test file in the Cypress application window to launch the test in a browser. Each time you save the test file, it will re-run itself.

Run tests headlessly with cypress run

To run your tests headlessly (e.g. when you're in CI), you'll want the run command:

$ rake cypress:run

Managing your test data

The tricky thing about browser tests is that they usually depend on some test data being available with which to exercise the app efficiently. Because cypress is a JavaScript-based tool and can't easily manipulate your Rails app directly, cypress-rails provides a number of hooks that you can use to manage your test data.

Here's what a config/initializers/cypress_rails.rb initializer might look like:

return unless Rails.env.test?

CypressRails.hooks.before_server_start do
  # Called once, before either the transaction or the server is started
end

CypressRails.hooks.after_transaction_start do
  # Called after the transaction is started (at launch and after each reset)
end

CypressRails.hooks.after_state_reset do
  # Triggered after `/cypress_rails_reset_state` is called
end

CypressRails.hooks.before_server_stop do
  # Called once, at_exit
end

(You can find an example initializer in this repo.)

The gem also provides a special route on the test server: /cypress_rails_reset_state. Each time it's called, cypress-rails will do two things at the beginning of the next request received by the Rails app:

• If CYPRESS_RAILS_TRANSACTIONAL_SERVER is enabled, roll back the transaction, effectively resetting the application state to whatever it was at the start of the test run

• Trigger any after_state_reset hooks you've configured (regardless of the transactional server setting)

This way, you can easily instruct the server to reset its test state from your Cypress tests like so:

beforeEach(() => {
  cy.request('/cypress_rails_reset_state')
})

(Remember, in Cypress, before is a before-all hook and beforeEach is run between each test case!)

Configuration

Environment variables

The cypress-rails gem is configured entirely via environment variables. If you find yourself repeating a number of verbose environment variables as you run your tests, consider invoking the gem from a custom script or setting your preferred environment variables project-wide using a tool like dotenv.

• CYPRESS_RAILS_DIR (default: Dir.pwd) the directory of your project
• CYPRESS_RAILS_HOST (default: "127.0.0.1") the hostname to bind to
• CYPRESS_RAILS_PORT (default: a random available port) the port to run the Rails test server on
• CYPRESS_RAILS_BASE_PATH (default: "/") the base path for all Cypress's requests to the app (e.g. via cy.visit()). If you've customized your baseUrl setting (e.g. in cypress.json), you'll need to duplicate it with this environment variable
• CYPRESS_RAILS_TRANSACTIONAL_SERVER (default: true) when true, will start a transaction on all database connections before launching the server. In general this means anything done during cypress open or cypress run will be rolled back on exit (similar to running a Rails System test)
• CYPRESS_RAILS_CYPRESS_OPTS (default: none) any options you want to forward to the Cypress CLI when running its open or run commands

Example: Running a single spec from the command line

It's a little verbose, but an example of using the above options to run a single Cypress test would look like this:

$ CYPRESS_RAILS_CYPRESS_OPTS="--spec cypress/integration/a_test.js" bin/rake cypress:run

Initializer hooks

before_server_start

Pass a block to CypressRails.hooks.before_server_start to register a hook that will execute before the server or any transaction has been started. If you use Rails fixtures, it may make sense to load them here, so they don't need to be re-inserted for each request

after_transaction_start

If there's any custom behavior or state management you want to do inside the transaction (so that it's also rolled back each time a reset is triggered), pass a block to CypressRails.hooks.after_transaction_start.

after_state_reset

Every time the test server receives an HTTP request at /cypress_rails_reset_state, the transaction will be rolled back (if CYPRESS_RAILS_TRANSACTIONAL_SERVER is enabled) and the after_state_reset hook will be triggered. To set up the hook, pass a block to CypressRails.hooks.after_state_reset.

before_server_stop

In case you've made any permanent changes to your test database that could pollute other test suites or scripts, you can use the before_server_stop to (assuming everything exits gracefully) clean things up and restore the state of your test database. To set up the hook, pass a block to CypressRails.hooks.before_server_stop.

Configuring Rails

Beyond the configuration options above, you'll probably also want to disable caching in your Rails app's config/environments/test.rb file, so that changes to your Ruby code are reflected in your tests while you work on them with rake cypress:open. (If either option is set to true, any changes to your Ruby code will require a server restart to be reflected as you work on your tests.)

To illustrate, here's what that might look like in config/environments/test.rb:

config.cache_classes = false
config.action_view.cache_template_loading = false

Setting up continuous integration

Circle CI

Nowadays, Cypress and Circle get along pretty well without much customization. The only tricky bit is that Cypress will install its large-ish binary to ~/.cache/Cypress, so if you cache your dependencies, you'll want to include that path:

version: 2
jobs:
  build:
    docker:
      - image: circleci/ruby:2.6-node-browsers
      - image: circleci/postgres:9.4.12-alpine
        environment:
          POSTGRES_USER: circleci
    steps:
      - checkout

      # Bundle install dependencies
      - type: cache-restore
        key: v1-gems-{{ checksum "Gemfile.lock" }}

      - run: bundle install --path vendor/bundle

      - type: cache-save
        key: v1-gems-{{ checksum "Gemfile.lock" }}
        paths:
          - vendor/bundle

      # Yarn dependencies
      - restore_cache:
          keys:
            - v1-yarn-{{ checksum "yarn.lock" }}
            # fallback to using the latest cache if no exact match is found
            - v1-yarn-

      - run: yarn install

      - save_cache:
          paths:
            - node_modules
            - ~/.cache
          key: v1-yarn-{{ checksum "yarn.lock" }}

      # Run your cypress tests
      - run: bin/rake cypress:run

Why use this?

Rails ships with a perfectly competent browser-testing facility called system tests which depend on capybara to drive your tests, most often with Selenium. All of these tools work, are used by lots of people, and are a perfectly reasonable choice when writing full-stack tests of your Rails application.

So why would you go off the Rails to use Cypress and this gem, adding two additional layers to the Jenga tower of testing facilities that Rails ships with? Really, it comes down to the potential for an improved development experience. In particular:

• Cypress's IDE-like open command provides a highly visual, interactive, inspectable test runner. Not only can you watch each test run and read the commands as they're executed, Cypress takes a DOM snapshot before and after each command, which makes rewinding and inspecting the state of the DOM trivially easy, something that I regularly find myself losing 20 minutes attempting to do with Capybara
• cypress open enables an almost REPL-like feedback loop that is much faster and more information dense than using Capybara and Selenium. Rather than running a test from the command line, seeing it fail, then adding a debug breakpoint to a test to try to manipulate the browser or tweaking a call to a Capybara API method, failures to be rather obvious when using Cypress and fixing it is usually as easy as tweaking a command, hitting save, and watching it re-run
• With very few exceptions, a Cypress test that works in a browser window will also pass when run headlessly in CI
• Cypress selectors are just jQuery selectors, which makes them both more familiar and more powerful than the CSS and XPath selectors offered by Capybara. Additionally, Cypress makes it very easy to drop into a plain synchronous JavaScript function for making more complex assertions or composing repetitive tasks into custom commands
• Cypress commands are, generally, much faster than analogous tasks in Selenium. Where certain clicks and form inputs will hang for 300-500ms for seemingly no reason when running against Selenium WebDriver, Cypress commands tend to run as fast as jQuery can select and fill an element (which is, of course, pretty fast)
• By default, Cypress takes a video of every headless test run, taking a lot of the mystery (and subsequent analysis & debugging) out of test failures in CI

Nevertheless, there are trade-offs to attempting this (most notably around Cypress's limited browser support and the complications to test data management), and I wouldn't recommend adopting Cypress and writing a bunch of browser tests for every application. But, if the above points sound like solutions to problems you experience, you might consider trying it out.

Code of Conduct

This project follows Test Double's code of conduct for all community interactions, including (but not limited to) one-on-one communications, public posts/comments, code reviews, pull requests, and GitHub issues. If violations occur, Test Double will take any action they deem appropriate for the infraction, up to and including blocking a user from the organization's repositories.