Plugins and Gems

Introduction

• Plugins is Rails extension mechanism that allows easy code sharing between Rails applications
• Plugins typically add application specific functionality or extend Rails in some way
• RubyGems is the standard Ruby packaging system and offer an alternative to plugins. Unlike plugins, RubyGems supports versioning and dependency management.

Where to Find Plugins

• There is a searchable database of plugins at agilewebdevelopment.com/plugins
• Most popular plugins have been moved from Subversion to Git and you’ll typically find the most recent version on Github.

Creating Plugins

• Generate your plugin with script/generate plugin
• The files install.rb and uninstall.rb are hooks that run on install/uninstall
• The init.rb file should load the plugins modules and classes that are kept under lib. In init.rb you have access to special variables config, directory, loaded_plugins, and name.
• Rake tasks are under tasks and tests under test. You can run the tests with rake in the plugin root directory.
• As of Rails 2.3 your plugin can contain app/models|controller|views|helpers directories as well as a config/routes.rb files and those will be loaded by Rails automatically.

A Few Recommended Plugins

• Will Paginate – paginate your lists of ActiveRecord objects
• Restful authentication or AuthLogic – user authentication
• Paperclip – file uploads
• Exception Notifier – receive email when errors occur in production (alternative: the Hoptoad service)
• Acts as State Machine – allow your models to act as a state machine with different states and transitions between these

The Plugin Module Inclusion Pattern

module MyPlugin
  def self.included(base)
    base.extend ClassMethods
    base.send :include, InstanceMethods
  end
  module InstanceMethods
    ...
  end
  module ClassMethods
    ...
  end
end

class ActiveRecord::Base
  include MyPlugin
end

A Word of Caution

• The general code quality of plugins varies a lot and is typically not as high as that of Rails itself
• Plugins that heavily override (monkey patch) core Rails methods can be error prone and make upgrading to newer Rails versions difficult
• Look for simple plugins that have a test suite and that have code that you can easily read and understand

RubyGems

• A gem is a packaged Ruby application or library. It has a name (e.g. rake) and a version (e.g. 0.4.16).
• Gems are managed on your computer using the gem command. You can install, remove, and query (amoung other things) gem packages using the gem command.
• RubyGems is the name of the project that developed the gem packaging system and the gem command.
• You can get RubyGems from the RubyForge repository.

Declaring Gem Dependencies

# In config/environment.rb:
config.gem "aws-s3", :lib => 'aws/s3', :version => "0.5.1" 
config.gem "money", :version => "1.7.1" 
config.gem "fastercsv", :version => "1.4.0" 
config.gem "hpricot", :version => '0.6', :source => "http://code.whytheluckystiff.net" 

Installing and Freezing Gems

# Install declared gem dependencies on the system (sudo gem install <gem-name>)
rake gems:install

# To avoid external libarary dependencies:
# 1. Copy declared gems to under RAILS_ROOT/vendor/gems
rake gems:unpack
# 2. Copy Rails itself to RAILS_ROOT/vendor/rails
rake rails:freeze:gems
# The Rails version is given by RAILS_GEM_VERSION in config/environment.rb.