Gem Dependencies

Rails plugins are great for many reasons, one being that they provide extra functionality without being an external dependency – they’re packaged right there with your application. Until recently, there was no way do programmatically define a Rails applications’ external gem dependencies and we were left with rolling our own gem dependency solutions.

That all changes with a nice way to define, and tie, gem dependencies to our Rails apps. In our environment.rb we have the following:

Rails::Initializer.run do |config|

# Require the latest version of haml
config.gem "haml"

# Require a specific version of chronic
config.gem "chronic", :version => '0.2.3'

# Require a gem from a non-standard repo
config.gem "hpricot", :source => "http://code.whytheluckystiff.net"

# Require a gem that needs to require a file different than the gem's name
# I.e. if you normally load the gem with require 'aws/s3' instead of
# require 'aws-s3' then you would need to specify the :lib option
config.gem "aws-s3", :lib => "aws/s3"
end
So great – when your app loads up it will automatically find and require each of the gems you’ve listed. But what if you’re on a system that doesn’t have all of these gems installed, or you’ve just deployed to a fresh environment? There’s now a rake task that will install all the referenced config.gems on your target system:

rake gems:install

Before running this you can see what gem dependencies your app has by running rake gems.

However, this still doesn’t package these gem dependencies with the app, it only refers to system-dependent gems. If you want to pull these gems into your application source you can do so with the new rake gems:unpack task:

# Unpack all gems to vendor/gems
rake gems:unpack

You can also unpack individual gems:

# Unpack only the hpricot gem to vendor/gems
rake gems:unpack GEM=hpricot

This will unpack the gem into the vendor/gems/hpricot-0.5 directory which is automatically searched as part of the config.gem startup.

Your deployment strategy can now choose to automatically install required gems in each target environment or just package gems as part of the application source.

Update: Just added is the ability to build gems that have native extensions:

rake gems:build

http://ryandaigle.com/articles/2008/4/1/what-s-new-in-edge-rails-gem-dependencies