Ruby on Rails Asset Pipeline


The asset pipeline provides a framework to concatenate and minify or compress JavaScript and CSS assets. It also adds the ability to write these assets in other languages and pre-processors such as CoffeeScript, Sass and ERB. It allows assets in your application to be automatically combined with assets from other gems. For example, jquery-rails includes a copy of jquery.js and enables AJAX features in Rails.

Rake tasks

By default sprockets-rails is shipped with the following rake tasks:

  • assets:clean[keep]: Remove old compiled assets
  • assets:clobber: Remove compiled assets
  • assets:environment: Load asset compile environment
  • assets:precompile: Compile all the assets named in config.assets.precompile

Manifest Files and Directives

In the assets initalizer (config/initializers/assets.rb) are a few files explicitly defined to be precompiled.

# Precompile additional assets.
#, application.scss, and all non-JS/CSS in app/assets folder are already added.
# Rails.application.config.assets.precompile += %w( search.js )

In this example the and application.scss are so called 'Manifest Files'. This files should be used to include other JavaScript or CSS assets. The following command are available:

  • require <path>: The require directive functions similar to Ruby's own require. It provides a way to declare a dependency on a file in your path and ensures it's only loaded once before the source file.
  • require_directory <path>: requires all the files inside a single directory. It's similar to path/* since it does not follow nested directories.
  • require_tree <path>: requires all the nested files in a directory. Its glob equivalent is path/**/*.
  • require_self: causes the body of the current file to be inserted before any subsequent require directives. Useful in CSS files, where it's common for the index file to contain global styles that need to be defined before other dependencies are loaded.
  • stub <path>: remove a file from being included
  • depend_on <path>: Allows you to state a dependency on a file without including it. This is used for caching purposes. Any changes made to the dependency file will invalidate the cache of the source file.

An application.scss file could look like:

 *= require bootstrap
 *= require_directory .
 *= require_self

Another example is the file. Here with including jquery and Turbolinks:

#= require jquery2
#= require jquery_ujs
#= require turbolinks
#= require_tree .

If you don't use CoffeeScript, but plain JavaScript, the syntax would be:

//= require jquery2
//= require jquery_ujs
//= require turbolinks
//= require_tree .

Basic Usage

There are two basic ways that the asset pipeline is used:

  1. When running a server in development mode, it automatically pre-processes and prepares your assets on-the-fly.

  2. In production mode, you’ll probably use it to pre-process, versionize, and compress and compile your assets. You can do so by running the following command:

    bundle exec rake assets:precompile