CSS Bundling for Rails

Use Tailwind CSS, Bootstrap, Bulma, PostCSS, or Dart Sass to bundle and process your CSS, then deliver it via the asset pipeline in Rails. This gem provides installers to get you going with the bundler of your choice in a new Rails application, and a convention to use app/assets/builds to hold your bundled output as artifacts that are not checked into source control (the installer adds this directory to .gitignore by default).

You develop using this approach by running the bundler in watch mode in a terminal with yarn build:css --watch (and your Rails server in another, if you're not using something like puma-dev). You can also use ./bin/dev, which will start both the Rails server and the CSS build watcher (along with a JS build watcher, if you're also using jsbundling-rails).

Whenever the bundler detects changes to any of the stylesheet files in your project, it'll bundle app/assets/stylesheets/application.[bundler].css into app/assets/builds/application.css. This build output takes over from the regular asset pipeline default file. So you continue to refer to the build output in your layout using the standard asset pipeline approach with <%= stylesheet_link_tag "application" %>.

When you deploy your application to production, the css:build task attaches to the assets:precompile task to ensure that all your package dependencies from package.json have been installed via yarn, and then runs yarn build:css to process your stylesheet entrypoint, as it would in development. This output is then picked up by the asset pipeline, digested, and copied into public/assets, as any other asset pipeline file.

This also happens in testing where the bundler attaches to the test:prepare task to ensure the stylesheets have been bundled before testing commences. If your test framework does not call the test:prepare Rake task, ensure that your test framework runs css:build to bundle stylesheets before testing commences. If your setup uses jsbundling-rails (ie, esbuild + tailwind), you will also need to run javascript:build.

That's it!

You can configure your bundler options in the build:css script in package.json or via the installer-generated tailwind.config.js for Tailwind or postcss.config.js for PostCSS.

You must already have node and yarn installed on your system. You will also need npx version 7.1.0 or later. Then:

1. Run ./bin/bundle add cssbundling-rails
2. Run ./bin/rails css:install:[tailwind|bootstrap|bulma|postcss|sass]

Or, in Rails 7+, you can preconfigure your new application to use cssbundling-rails for bootstrap, bulma or postcss with rails new myapp --css [bootstrap|bulma|postcss].

If you're already relying on Node to process your JavaScript, this gem is the way to go. But if you're using the default import map setup in Rails 7+, you can avoid having to deal with Node at all by using the standalone versions of Tailwind CSS and Dart Sass that are available through tailwindcss-rails and dartsass-rails. It's simpler, fewer moving parts, and still has all the functionality.

In Rails 7+, you can preconfigure your new application to use tailwindcss-rails and dartsass-rails with rails new myapp --css [tailwind|sass].

If you want to use @import statements as part of your Tailwind application.js file, you need to configure Tailwind to use postcss and then postcss-import. But you should also consider simply referring to your other CSS files directly, instead of bundling them all into one big file. It's better for caching, and it's simpler to setup. You can refer to other CSS files by expanding the stylesheet_link_tag in application.html.erb like so: <%= stylesheet_link_tag "application", "other", "styles", "data-turbo-track": "reload" %>.

Some CSS packages use new CSS features that are not supported by the default SassC rails integration that previous versions of Rails used. If you hit such an incompatibility, it'll likely be in the form of a SassC::SyntaxError when running assets:precompile. The fix is to bundle remove sass-rails (or sassc-rails, if you were using that).

A common issue is that your repository does not contain the output directory used by the build commands. You must have app/assets/builds available. Add the directory with a .gitkeep file, and you'll ensure it's available in production.

This might happen if your Gemfile.lock contains the legacy sassc-rails, which might be need while progressively migrating your project, or which might be a transitive dependency of a gem your project depends on and over which you have no control. In this case, prevent Sprockets from bundling the CSS on top of the bundling already performed by this gem. Make sure do this for all environments, not only production, otherwise your test suite may fail.

# config/initializers/assets.rb
Rails.application.config.assets.css_compressor = nil

Watch out - if you precompile your files locally, those will be served over the dynamically created ones you expect. The solution:

rails assets:clobber 

Use an @import statement and path to a specific stylesheet, omitting the node_modules/ segment and the file's extension. For example:

/* Desired file is at at node_modules/select2/dist/css/select2.css */
@import "select2/dist/css/select2";

CSS Bundling for Rails is released under the MIT License.