application.rb

if defined?(Bundler)
  bundle_groups = {
    :assets => %w{development test},
    :app => %w{test}, # load web gems for controllers
    :debug => %w{development test}, # load code debugger
    :ci => %w{development test} # loads test dependencies
  }
  Bundler.require *Rails.groups(bundle_groups)
end

config.ru

# ensure rack environment loads app dependencies
groups = ENV['RAILS_GROUPS'].to_s.split(',')
groups << 'app'
ENV['RAILS_GROUPS'] = groups.compact.uniq.join(',')

require ::File.expand_path('../config/environment',  __FILE__)

deploy.rb

require "bundler/capistrano"

# exclude non-production dependencies from production servers
set :bundle_without, [:development, :test, :darwin, :debug, :deploy, :ci, :ct]

Gemfile

# VERSIONING
# all production gems should use STRICT version dependencies ex: "1.0.0"
# development and test gems can you LOOSE version dependencies ex: "~> 1.0.0"

source 'http://rubygems.org'

# list gems for *all* environments/groups here
# ex: rails, mysql2, rails-console-tweaks

# list gems for frontend app servers here
# use for rack components and other "web" specific dependencies
# see config.ru to autoload these dependencies
# ex: rack-ssl-enforcer, detect_browser_os
group :app do
end

# list gems for resque background workers here
# see script to start resque jobs to autoload these dependencies
# ex: resque-ensure-connected
group :worker do
end

# list gems used to build asset packages here
# see rails capistrano tasks to autoload these dependencies
# ex: closure-compiler, sass-rails, jquery-rails
group :assets do
end

# list dependencies for rails/irb console here
# NOTE: the rails-console-tweaks gem must be in the default production group
# for these gems to be loaded
# ex: hirb, awesome_print
group :console do
end

# list gems that are used in both development *and* test environments here
# gems with rake tasks used for testing must be here as well
# ex: rspec-rails
group :development, :test do
end

# list gems used for development environment here
# ex: mailcatcher, email_preview
group :development do
end

# list gems used for debugging application here
# ex: pry
group :debug do
end

# list gems used for continuous testing workflow here
# ex: guard
group :ct do
end

# list gems used for deploying application here
# ex: capistrano
group :deploy do
end

# list native gems only usable for mac developers here
# ex: growl
group :darwin do
end

# list gems used for unit testing here
# ex: rspec, webmock, webrat, shoulda
group :test do
end

# list gems used for continuous integration testing here
# ex: 
group :ci do
end