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TOTD #6: Difference between Ruby Gem and Rails Plugin

Posted by arungupta on September 4, 2007 at 11:09 AM PDT

As a newbie to
Rails, "Gem" and "Plugin" terminology
was slightly confusing to me. Specifically I saw the terms being used
interchangeably sometimes. This
TOTD highlights the
difference between Ruby Gem and
Rails Plugin
.

Ruby Gem and Plugin are both packaging systems - the main difference is the
scope in which they are effective. The common differences are highlighted in the table
below.

Topic Ruby Gem
Rails Plugin
Definition A Gem is a packaged Ruby application using the packaging
system defined by RubyGems.
A Rails plugin is a
packaged Ruby application that extends the core
Rails framework.
Example Has a
name (e.g. rake) and a version (e.g. 0.4.16).
Rails is also installed as a Gem.
Has a name (e.g.
Goldspike)
and a version (e.g. 1.1.1).
Scope Installed in the Ruby or JRuby installation
and is available to all applications run using that interpreter.
Installed in a specific Rails app only.
Packaging System Rails gem is installed in
		jruby-1.0\lib\ruby\gems\1.8\gems\rails-1.2.3
as:
<DIR>        
bin

<DIR>        builtin

      68,465 CHANGELOG

<DIR>        configs

<DIR>        dispatches

<DIR>        doc

<DIR>        environments

307          fresh_rakefile

<DIR>        helpers

<DIR>        html

<DIR>        lib

       1,072 MIT-LICENSE

      11,969 Rakefile

       8,001 README

The lib directory contains all the gem source code.

Goldspike plugin is installed in
		vendor\plugins\rails-integration
directory of the application as:
      7,089 build.xml

      1,141 LICENSE.txt

<DIR>       plugins

      6,675 pom.xml

      1,447 README

<DIR>       samples
plugins/goldspike directory consists of

         24 init.rb

         25 install.rb

<DIR>       lib

        549 Rakefile

        536 README

<DIR>       tasks

<DIR>       test

The lib directory contains all the plugin source code.

Management Gems are managed on your computer using the gem command. You can install, remove and query gem
packages using the gem command.
Plugins can be installed using after creating a Rails application and then
invoking the command




ruby script\plugin install [url or name of plugin]
Load Path gem command adds the lib
directory of your gem to the load path of Ruby.
Rails adds the lib directory of plugin in
your application's load path.
Specific Features  
  • Bundled Rake tasks that get automatically loaded into your
    Rakefile
  • An installation hook (install.rb)
  • An application initialization hook for the plugin to inject
    itself (init.rb)
  • ability to bundle additional generators (for the
    script/generate script)
Publish Format A Gem may be published as Plugin, for
example ActiveRecord-JDBC.
A Plugin cannot be published as Gem.

Here are some useful entries (with numerous pointers on screencasts, blogs,
tips & extensive feature sets) to get you started with JRuby:

Are there any other main differences that I missed ?

Please leave suggestions on other TOTD that you'd like to see. A complete
archive is available here.

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