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In Rails, plugins are special libraries that add functionality to the Rails framework within the scope of the application they are installed under or the Rails application itself. The Rails application can then use the functionality added by the plugin. There are hundreds of Rails plugins, and the number is growing all the time as Rails developers come up with new ideas. You can get plugins to make it easy to create graphs from your application, to add tagging functionality to your code, and even to add large chunks of functionality, such as an entire authentication system, to your app. Installing a plugin is even easier than installing a gem. You use the script/plugin script, like so:

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If you add the occupation camper, you get the following output: Description of Throatwobbler Mangrove Age: 42 Occupation: camper The code is intuitive, but a bit inefficient (although the main concern here is really code simplicity) It has to look up the key 'occupation' twice once to see whether the key exists (in the condition) and once to get the value (to print it out) An alternative definition is def describePerson(person): print 'Description of', person['name'] print 'Age:', person['age'] try: print 'Occupation:', person['occupation'] except KeyError: pass Here the function simply assumes that the key 'occupation' is present If you assume that it normally is, this saves some effort: The value will be fetched and printed no extra fetch to check whether it is indeed there If the key doesn t exist, a KeyError exception is raised, which is trapped by the except clause.

Note Like gem, the plugin script supports operations such as install, remove, and update to perform the relevant action upon the plugin. You can learn all about the various supported operations by running ruby script/plugin h.

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The URL for the plugins you want to install is provided by the author of that plugin on his or her Web page, or wherever you learn about the plugin. Here s a demonstration of installing a plugin called acts_as_commentable:

ruby script/plugin install http://juixe.com/svn/acts_as_commentable/

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You may also find try/except useful when checking whether an object has a specific attribute or not Let s say you want to check whether an object has a write attribute, for example Then you could use code like this: try: objwrite except AttributeError: print 'The object is not writeable' else: print 'The object is writeable' Here the try clause simply accesses the attribute without doing anything useful with it If an AttributeError is raised, the object doesn t have the attribute; otherwise, it has the attribute This is a natural alternative to the getattr solution introduced in 7 (in the section Interfaces and Introspection ) Which one you prefer is largely a matter of taste Indeed, getattr is internally implemented in exactly this way: It tries to access the attribute and catches the AttributeError that this attempt may raise.

./acts_as_commentable/MIT-LICENSE ./acts_as_commentable/README ./acts_as_commentable/init.rb ./acts_as_commentable/install.rb ./acts_as_commentable/lib/acts_as_commentable.rb ./acts_as_commentable/lib/comment.rb ./acts_as_commentable/tasks/acts_as_commentable_tasks.rake ./acts_as_commentable/test/acts_as_commentable_test.rb

The result from running script/plugin shows which files were added to the project. Plugins are stored in the vendor/plugins folder, so this plugin is now found at vendor/ plugins/acts_as_commentable. When your Rails application next starts, the acts_as_commentable plugin will be loaded automatically (as all plugins located in vendor/plugins are), and you can use its functionality within your application. Plugin usage varies significantly from plugin to plugin, but good places to find plugins to use are http://www.agilewebdevelopment.com/plugins and http://plugins. radrails.org/.