ruby on rails - How to add Mac-specific gems to bundle on Mac but not on Linux?

Question

I'm developing a Rails app on a Mac, and I'm new to testing, so I just added these gems to my Gemfile:

group :test, :development do
  gem 'rspec-rails'     
  gem 'rb-fsevent'
  gem 'growl'
end

But my production server runs Linux, so even if they aren't grouped in :production, bundler (v1.0.21) still attempts to install them. ...and fails, obviously!
extconf.rb:19:in '<main>': Only Darwin (Mac OS X) systems are supported (RuntimeError)

Setting RAILS_ENV to production before running bundle install doesn't work.

It worked by running bundle install --without development test, but how can these gems be taken into consideration by bundler only based on your OS?

---

Edit: The bundler wiki provides details on how to use platform as a parameter.
The same solution is given in bundler issue #663, so I tried:

group :test, :development do
  gem 'rspec-rails'     
  platforms :darwin do
    gem 'rb-fsevent'
    gem 'growl'
  end 
end

bundle install does not work, but even if we go back to square one and do
bundle install --without darwin, the result is 'darwin' is not a valid platform.
The available options are: [:ruby, :ruby_18, :ruby_19, :mri, :mri_18, :mri_19, :rbx, :jruby, :mswin, :mingw, :mingw_18, :mingw_19]

---

Any other (elegant) approaches?

Answer 1

The Bundler wiki has a method that adds all gems to Gemfile.lock regardless of platform. It sets require => false depending on the system so you don't need to be able to actually run the gems:

gem 'rb-inotify', :require => RUBY_PLATFORM.include?('linux') && 'rb-inotify'

And they provide helper methods to make this clean:

def os_is(re)
  RbConfig::CONFIG['host_os'] =~ re
end

gem 'rb-fsevent', "~> 0.9.3", platforms: :ruby, install_if: os_is(/darwin/)
gem 'rb-inotify', "~> 0.8.8", platforms: :ruby, install_if: os_is(/linux/)
gem 'wdm',        "~> 0.1.0", platforms: [:mswin, :mingw. :x64_mingw], install_if: os_is(/mingw|mswin/i)

On my Windows 7 x64 system running Ubuntu 12.04 in a Vagrant VM, this worked fine, but the :platforms setting was required - the Linux bundler choked on the 'win32console' gem:

Console.c:1:21: fatal error: windows.h: No such file or directory