Create an :assets group in the Gemfile.

This group is required by default only on development and test
(you can change it on config/application.rb).

`rake assets:precompile` will automatically add the assets group
to Rails.groups (and consequently Bundler.require) and should work
transparently.

railties/lib/rails/generators/app_base.rb

@@ -164,15 +164,15 @@ def gem_for_database
         end
       end
 
-      def gem_for_ruby_debugger
+      def ruby_debugger_gemfile_entry
         if RUBY_VERSION < "1.9"
           "gem 'ruby-debug'"
         else
           "gem 'ruby-debug19', :require => 'ruby-debug'"
         end
       end
 
-      def gem_for_turn
+      def turn_gemfile_entry
         unless RUBY_VERSION < "1.9.2" || options[:skip_test_unit]
           <<-GEMFILE.strip_heredoc
             group :test do
@@ -183,7 +183,7 @@ def gem_for_turn
         end
       end
 
-      def gem_for_javascript
+      def javascript_gemfile_entry
         "gem '#{options[:javascript]}-rails'" unless options[:skip_javascript]
       end
 

railties/lib/rails/generators/rails/app/templates/Gemfile

@@ -5,13 +5,17 @@ source 'http://rubygems.org'
 <%= database_gemfile_entry -%>
 
 <%= "gem 'jruby-openssl'\n" if defined?(JRUBY_VERSION) -%>
-# Asset template engines
 <%= "gem 'json'\n" if RUBY_VERSION < "1.9.2" -%>
-gem 'sass-rails', "~> 3.1.0.rc"
-gem 'coffee-script'
-gem 'uglifier'
 
-<%= gem_for_javascript %>
+# Gems used only for assets and not required
+# in production environments by default.
+group :assets do
+  gem 'sass-rails', "~> 3.1.0.rc"
+  gem 'coffee-script'
+  gem 'uglifier'
+end
+
+<%= javascript_gemfile_entry %>
 
 # Use unicorn as the web server
 # gem 'unicorn'
@@ -20,6 +24,6 @@ gem 'uglifier'
 # gem 'capistrano'
 
 # To use debugger
-# <%= gem_for_ruby_debugger %>
+# <%= ruby_debugger_gemfile_entry %>
 
-<%= gem_for_turn -%>
+<%= turn_gemfile_entry -%>

railties/lib/rails/generators/rails/app/templates/config/application.rb

@@ -11,9 +11,10 @@
 <%= comment_if :skip_test_unit %> require "rails/test_unit/railtie"
 <% end -%>
 
-# If you have a Gemfile, require the gems listed there, including any gems
-# you've limited to :test, :development, or :production.
-Bundler.require(:default, Rails.env) if defined?(Bundler)
+# If you have a Gemfile, require the default gems, the ones in the
+# current environment and also include :assets gems if in development
+# or test environments.
+Bundler.require *Rails.groups(:assets => %w(development test)) if defined?(Bundler)
 
 module <%= app_const_base %>
   class Application < Rails::Application

railties/lib/rails/generators/rails/plugin_new/templates/Gemfile

@@ -7,9 +7,8 @@ source "http://rubygems.org"
 <% end -%>
 
 <% if mountable? -%>
-<%= gem_for_javascript -%>
+<%= javascript_gemfile_entry -%>
 <% end -%>
 
-if RUBY_VERSION < '1.9'
-  gem "ruby-debug", ">= 0.10.3"
-end
+# To use debugger
+# <%= ruby_debugger_gemfile_entry %>

railties/lib/rails/tasks/assets.rake

@@ -1,11 +1,16 @@
 namespace :assets do
   desc "Compile all the assets named in config.assets.precompile"
-  task :precompile => :environment do
-    # Give assets access to asset_path
-    Sprockets::Helpers::RailsHelper
+  task :precompile do
+    if ENV["RAILS_GROUPS"].to_s.empty?
+      ENV["RAILS_GROUPS"] = "assets"
+      Kernel.exec $0, *ARGV
+    else
+      Rake::Task["environment"].invoke
+      Sprockets::Helpers::RailsHelper
 
-    assets = Rails.application.config.assets.precompile
-    Rails.application.assets.precompile(*assets)
+      assets = Rails.application.config.assets.precompile
+      Rails.application.assets.precompile(*assets)
+    end
   end
 
   desc "Remove compiled assets"

railties/test/application/assets_test.rb

@@ -1,8 +1,9 @@
 require 'isolation/abstract_unit'
+require 'active_support/core_ext/kernel/reporting'
 require 'rack/test'
 
 module ApplicationTests
-  class RoutingTest < Test::Unit::TestCase
+  class AssetsTest < Test::Unit::TestCase
     include ActiveSupport::Testing::Isolation
     include Rack::Test::Methods
 
@@ -34,6 +35,17 @@ def app
       assert_match "alert()", last_response.body
     end
 
+    test "assets are compiled properly" do
+      app_file "app/assets/javascripts/application.js", "alert();"
+
+      capture(:stdout) do
+        Dir.chdir(app_path){ `bundle exec rake assets:precompile` }
+      end
+
+      file = Dir["#{app_path}/public/assets/application-*.js"][0]
+      assert_equal "alert();\n", File.read(file)
+    end
+
     test "does not stream session cookies back" do
       app_file "app/assets/javascripts/demo.js.erb", "<%= :alert %>();"
 

railties/test/isolation/abstract_unit.rb

@@ -239,6 +239,10 @@ def app_file(path, contents)
       end
     end
 
+    def remove_file(path)
+      FileUtils.rm_rf "#{app_path}/#{path}"
+    end
+
     def controller(name, contents)
       app_file("app/controllers/#{name}_controller.rb", contents)
     end