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We recently suffered from asset pipeline performance issues. In order to boost deploying, we decide to switch to capistrano-local-precompile strategy.

However, local asset compiling in Rails 4 is blazing fast, but in Rails 3 is not.

And much nightmare is if we can skip compiling prcoess if assets weren't changed when we compiling at remote, but we need to compile asset everytime if we choose to compile locally.

So we comes out this crazy idea: How about running Rails 4 asset pipeline in Rails 3 project.

Here is how:

1. modify Gemfile

group :assets do
  gem 'sprockets', '2.2.2.backport2'
  gem 'sprockets-rails', :require => 'sprockets/railtie', :git => "git@github.com:logdown/sprockets-rails.git"

  gem 'sass-rails',   '~> 4.0.1', :git => "git@github.com:logdown/sass-rails.git"
  gem "compass-rails", "~> 1.1.2", :git => "git@github.com:logdown/compass-rails.git"
 
  gem 'coffee-rails', '~> 3.2.1'

  gem 'uglifier', '>= 1.0.3'
end

2. edit config/enviorments/production.rb

  # Disable Rails's static asset server (Apache or nginx will already do this)

  config.serve_static_assets = true

  # Compress JavaScripts and CSS

  config.assets.compress = true


  # Generate digests for assets URLs

  config.assets.digest = true
  
  config.assets.js_compressor = :uglifier
  config.assets.css_compressor = :sass

3. change config/deploy.rb

You should install capistrano-local-precompile first, and override precompile command. It's d because the precompile machism in different versions are different.

set :precompile_cmd, "bundle exec rake RAILS_ENV=production RAILS_GROUPS=assets assets:precompile"

Behinde the scene

Basically we are doing serveral things:

1. make sprockets-rails edge running with sprockets 2.2
2. make sass-rails edge running under rails 3
3. make compass-rails edge running with rails 4 strategy

to resolve asset_paths, SASS resolver issue...etc.

Warning

Before doing this, make sure you really understand what your are doing. And it exists many different between sprockets 1 & 2.