Keeping locally precompiled assets out of the way on Heroku

With the Rails 3.1 Asset Pipeline, there’s the rake assets:precompile command which precompiles assets into public/assets along with a manifest.yml file which rails uses to put the paths to the assets in its templates. Precompiling generates a few files, which can clutter commit logs if they’re committed. For this reason I might add public/assets to my .gitignore and build them on the server, rather than committing them to git and uploading them. Or, if the server had trouble compiling them, I might precompile them locally, and scp them up rather than committing them to git, because they can make a lot of noise in my git logs.

On Heroku, though, git is the only way to deploy. So if I’m having trouble having heroku precompile them, I need to commit them and push them via git. This could clutter up my commit log.

There is a way around this, though: use a branch! If I create a branch just for heroku, and just commit them to the branch, I can keep my commit logs free of changes to public/assets.

Here is a sketch of this process:

  1. Add public/assets to my .gitignore and commit it
  2. Create a heroku branch with git checkout -b heroku
  3. remove public/assets from my .gitignore and commit it. This just affects my heroku branch.
  4. Run RAILS_ENV=production rake assets:precompile there.
  5. Run git add . and git commit -m 'precompile assets' to commit the files to my heroku branch
  6. Run git push heroku heroku:master to push my heroku branch to heroku as master (heroku only uses the master branch)
  7. Run git checkout master and continue development on my master branch.

A deploy rake script could be used to automate this.

The heroku branch wouldn’t need to be pushed to github, because it doesn’t contain any information that can’t be regenerated. The only reason the history would need to be preserved is so it can be cleanly pushed to the heroku app. Fortunately, if the local repo was lost, it’s possible to pull from the heroku repository. Heroku shouldn’t be used as a place to store git repositories because the lifecycle for a heroku app is often different than that of a repository. For this purpose, though, the usefulness of the old assets commits is tied to the life of the Heroku app so it works out well.

Adding Compass to a Heroku App

I started building a Rails 3.1 app that runs on Heroku using the Cedar stack. In this app I really want to take advantage of the Rails 3.1 Asset Pipeline. I’m using CoffeeScript, Sass, and Compass. The former two come with Rails 3.1 but I needed to add Compass myself. I used the instructions here, but used the latest version of Compass because it now works, and I added a check in the initializer to see if sass was loaded. The check in the initializer is needed because Heroku only runs with the asset gems from the Gemfile when it compiles the assets, and it only compiles the assets when a new slug is loaded. So what happened is that the Rails 3.1 initializer, sass.rb, tried setting a configuration parameter in sass when sass wasn’t loaded.

Here’s the relevant code. The asset section of my Gemfile now contains compass:

group :assets do
  gem 'sass-rails', "  ~> 3.1.0"
  gem 'coffee-rails', "~> 3.1.0"
  gem 'uglifier'
  gem 'compass'

…and I added config/initializers/sass.rb:

if Rails.configuration.respond_to?(:sass)
  Rails.configuration.sass.tap do |config|
    config.load_paths << "#{Gem.loaded_specs['compass'].full_gem_path}/frameworks/compass/stylesheets"

It uses respond_to? to see if it there is a sass configuration object.

Right now I’m using Compass to set a border radius.