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Bundler.setup

Configure the load path so all dependencies in your Gemfile can be required 
require 'rubygems'
require 'bundler/setup'
require 'nokogiri'
Only add gems from specified groups to the load path. If you want the gems in the default group, make sure to include it 
require 'rubygems'
require 'bundler'
Bundler.setup(:default, :ci)
require 'nokogiri'
Learn More: Groups

Compatibility

Ruby 2.0 and RubyGems 2.0 both require Bundler 1.3 or later. If you have questions about compatibility between Bundler and your system, please check the compatibility list. Learn More: Compatibility

Setting Up Your Application to Use Bundler

Bundler makes sure that Ruby can find all of the gems in the Gemfile (and all of their dependencies). If your app is a Rails 3 app, your default application already has the code necessary to invoke bundler. If it is a Rails 2.3 app, please see: Setting up Bundler in Rails 2.3
For another kind of application (such as a Sinatra application), you will need to set up bundler before trying to require any gems. At the top of the first file that your application loads (for Sinatra, the file that calls require 'sinatra'), put the following code: 
require 'rubygems'
require 'bundler/setup'
This will automatically discover your Gemfile, and make all of the gems in your Gemfile available to Ruby (in technical terms, it puts the gems "on the load path"). You can think of it as an adding some extra powers to require 'rubygems'.
Now that your code is available to Ruby, you can require the gems that you need. For instance, you can require 'sinatra'. If you have a lot of dependencies, you might want to say "require all of the gems in my Gemfile". To do this, put the following code immediately following require 'bundler/setup': 
Bundler.require(:default)
For our example Gemfile, this line is exactly equivalent to: 
require 'rails'
require 'rack-cache'
require 'nokogiri'
Astute readers will notice that the correct way to require the rack-cache gem is require 'rack/cache', not require 'rack-cache'. To tell bundler to use require 'rack/cache', update your Gemfile: 
source 'https://rubygems.org'

gem 'rails', '3.0.0.rc'
gem 'rack-cache', :require => 'rack/cache'
gem 'nokogiri', '~> 1.4.2'
For such a small Gemfile, we'd advise you to skip Bundler.require and just require the gems by hand (especially given the need to put in a :require directive in the Gemfile). For much larger Gemfiles, using Bundler.require allows you to skip repeating a large stack of requirements.
Edit this document on GitHub if you caught an error or noticed something was missing.