How to manage groups of gems

Grouping your dependencies allows you to perform operations on the entire group.
# These gems are in the :default group
gem 'nokogiri'
gem 'sinatra'

gem 'wirble', group: :development

group :test do
  gem 'faker'
  gem 'rspec'
end

group :test, :development do
  gem 'capybara'
  gem 'rspec-rails'
end

gem 'cucumber', group: [:cucumber, :test]
Configure bundler so that subsequent `bundle install` invokations will install all gems, except those in the listed groups. Gems in at least one non-excluded group will still be installed.
$ bundle config set --local without test development
Require the gems in particular groups, noting that gems outside of a named group are in the :default group
Bundler.require(:default, :development)
Require the default gems, plus the gems in a group named the same as the current Rails environment
Bundler.require(:default, Rails.env)
Restrict the groups of gems that you want to add to the load path. Only gems in these groups will be require'able. Note though that `Bundler.setup` can be called only once, all subsequent calls are no-op. In particular, since running a script through `bundle exec` already calls `Bundler.setup`, any later calls inside your user code will be ignored. In order to control the groups that are loaded by `bundle exec` you can use the `BUNDLE_WITH` and `BUNDLE_WITHOUT` configurations.
require 'rubygems'
require 'bundler'
Bundler.setup(:default, :ci)
require 'nokogiri'
Learn More: Bundler.setup

Optional groups and BUNDLE_WITH

Mark a group as optional using group :name, optional: true do and then opt into installing an optional group with bundle config set --local with name.

Grouping your dependencies

You'll sometimes have groups of gems that only make sense in particular environments. For instance, you might develop your app (at an early stage) using SQLite but deploy it using mysql2 or pg. In this example, you might not have MySQL or Postgres installed on your development machine and want bundler to skip it. To do this, you can group your dependencies:
source 'https://rubygems.org'

gem 'rails', '3.2.2'
gem 'rack-cache', require: 'rack/cache'
gem 'nokogiri', '~> 1.4.2'

group :development do
  gem 'sqlite3'
end

group :production do
  gem 'pg'
end
Now, in development, you can instruct bundler to skip the production group:
$ bundle config set --local without production
Bundler stores the flag in APP_ROOT/.bundle/config and the next time you run `bundle install`, it will skip production gems. Similarly, when you require `bundler/setup`, Bundler will ignore gems in these groups. You can see all of the settings that Bundler saved there by running bundle config, which will also print out global settings (stored in ~/.bundle/config) and settings set via environment variables. For more information on configuring Bundler, please see: bundle config

You can also specify which groups to automatically require through the parameters to Bundler.require. The :default group includes all gems not listed under any group. If you call Bundler.require(:default, :development), bundler will require all the gems in the :default group as well as the gems in the :development group.

By default, a Rails generated app calls Bundler.require(:default, Rails.env) in your application.rb, which links the groups in your Gemfile to the Rails environment. If you use other groups (not linked to a Rails environment), you can add them to the call to Bundler.require if you want them to be automatically required.

Remember that you can always leave groups of gems out of Bundler.require and then require them manually using Ruby's require at the appropriate place in your app. You might do this because requiring a certain gem takes some time and you don't need it every time you boot your application.

Edit this document on GitHub if you caught an error or noticed something was missing.