tags: rails
Originally Published: 2020-09-26

I recently switched a project over to using Rails 6’s built-in parallel tests, by setting parallelize(workers: :number_of_processors) in tests/test_helper.rb

While this seemed to work fine locally, tests on CircleCI would randomly fail with multiple errors like the following:

	Webpacker can't find application.js in /home/circleci/repo/public/packs-test/manifest.json. Possible causes:
	1. You want to set webpacker.yml value of compile to true for your environment
		 unless you are using the `webpack -w` or the webpack-dev-server.
	2. webpack has not yet re-run to reflect updates.
	3. You have misconfigured Webpacker's config/webpacker.yml file.
	4. Your webpack configuration is not creating a manifest.
	Your manifest contains:

Ultimately, this seemed to be the result of a race condition when using the default Rails webpacker test environment configuration, which compiles packs on request.

I solved this by editing my config/webpacker.yml to put the test environment block below production, defining production as a YAML alias, and inheriting from it for the test environment, so that assets for test are precompiled once:

production: &production
  <<: *default

  # Production depends on precompilation of packs prior to booting for performance.
  compile: false

  # Extract and emit a css file
  extract_css: true

  # Cache manifest.json for performance
  cache_manifest: true

  <<: *production

  # Compile test packs to a separate directory
  public_output_path: packs-test

Note that this means you’ll need to run e.g. RAILS_ENV=test bundle exec rails assets:precompile before running your tests. I added the following to my Rakefile so that for local development, the default task (run when I just run rake) will precompile assets and then run tests:


	task :default do