The How and Why of Bundler Groups

Since version 0.9, Bundler has had a feature called "groups". The purpose of this feature is to allow you to specify groups of dependencies which may be used in certain situations, but not in others.

For instance, you may use ActiveMerchant only in production. In this case, you could say:

group :production do  
  gem "activemerchant"
end  

Specifying groups allows you to do two things. First, you can install the gems in your Gemfile, minus specific groups. For instance, Rails puts mysql and pg in a database group so that if you're just working on ActionPack, you can bundle install --without db and run the ActionPack tests without having to worry about getting the gems installed.

Second, you can list specific groups to autorequire using Bundler.require. By default, Bundler.require requires all the gems in the default group (which is all the gems that have no explicit group). You can also say Bundler.require(:default, :another_group) to require specific groups.

Note the difference between these operations: bundle install is opt-out, while Bundler.require is opt-in. This is because the common usage of groups is to specify gems for different environments (such as development, test and production) and you shouldn't need to specify that you want the "development" and "test" gems just to get up and running. On the other hand, you don't want your test dependencies loaded in development or production.

It is also worth noting that all gems that you installed (i.e. not the ones that you excluded at install time with --without) will be available to require. This has no effect unless you actually require them. This means that in development mode, if you explicitly require rspec, it will work.

Rails 3 defaults to mapping groups to environment names, and explicitly autorequiring the implicit default group and the group named the same as the current environment. For example, in development mode, Rails will require the default group and the development group. The code that does this is in your application.rb:

Bundler.require(:default, Rails.env) if defined?(Bundler)  

Consistency

In order to ensure consistency across all environments, bundler resolves the dependencies of your application using the gems listed in all groups, even if you specify --without. This means that while you can skip installing the gems listed in the production group by saying --without production, bundler will still download and examine the gems in order to properly resolve all dependencies.

As a result, the dependencies you install in development mode and test with will be compatible with the gems in other environments. In essence, this policy ensures that if your tests pass and run in development, your app will not fail to run in production because the dependencies resolved differently.

Multiple Inconsistent Configurations

Sometimes, especially when developing gems for wider use, you want to test your code against multiple incompatible configurations. At first glance, you might think that you could use groups for this case, but as described above, groups are designed for cases where all of the gems are compatible, but you don't always want to have to install them in all situations.

Instead, use multiple Gemfiles, one for each incompatible configuration. When installing, do bundle install --gemfile Gemfile.rails2. This will tell Bundler to use Gemfile.rails2 rather than the default Gemfile. As in all cases in Bundler, you can also specify this option globally with an environment variable (BUNDLE_GEMFILE).