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Rails 3 comes with baked in support with bundler.

Using Bundler with Rails 3

Install Rails as you normally would. Use sudo if you would normally use sudo to install gems.
$ gem install rails
We recommend using rvm for dependable Ruby installations, especially if you are switching between different versions of Ruby
Generate a Rails app as usual
$ rails new myapp
$ cd myapp

Run the server. Bundler is transparently managing your dependencies!
$ rails server

Add new dependencies to your Gemfile as you need them.
gem 'nokogiri'
gem 'geokit'

If you want a dependency to be loaded only in a certain Rails environment, place it in a group named after that Rails environment
group :test do
  gem 'rspec'
  gem 'faker'
end

You can place a dependency in multiple groups at once as well
group :development, :test do
  gem 'wirble'
  gem 'ruby-debug'
end
Learn More: Groups
After adding a dependency, if it is not yet installed, install it
$ bundle install
This will update all dependencies in your Gemfile to the latest versions that do not conflict with other dependencies
Edit this document on GitHub if you caught an error or noticed something was missing.