Question

I don’t understand what exactly is going on with this group, and what bundler is doing with it. Is it only loaded in dev mode? What if I want to make a new environment type, how should I handle this group? Etc.

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

Answer 1

The code that handles :assets group placed in config\application.rb. In rails 3.1 it is:

if defined?(Bundler)
  # If you precompile assets before deploying to production, use this line
  Bundler.require *Rails.groups(:assets => %w(development test))
  # If you want your assets lazily compiled in production, use this line
  # Bundler.require(:default, :assets, Rails.env)
end