The Basics of Creating Rails Plugins

A Rails plugin is either an extension or a modification of the core framework. Plugins provide:

a way for developers to share bleeding-edge ideas without hurting the stable code base
a segmented architecture so that units of code can be fixed or updated on their own release schedule
an outlet for the core developers so that they don’t have to include every cool new feature under the sun

After reading this guide you should be familiar with:

Creating a plugin from scratch
Writing and running tests for the plugin
Storing models, views, controllers, helpers and even other plugins in your plugins
Writing generators
Writing custom Rake tasks in your plugin
Generating RDoc documentation for your plugin
Avoiding common pitfalls with init.rb

This guide describes how to build a test-driven plugin that will:

Extend core ruby classes like Hash and String
Add methods to ActiveRecord::Base in the tradition of the acts_as plugins
Add a view helper that can be used in erb templates
Add a new generator that will generate a migration
Add a custom generator command
A custom route method that can be used in routes.rb

For the purpose of this guide pretend for a moment that you are an avid bird watcher. Your favorite bird is the Yaffle, and you want to create a plugin that allows other developers to share in the Yaffle goodness. First, you need to get setup for development.

1. Setup

1.1. Create the basic app

The examples in this guide require that you have a working rails application. To create a simple rails app execute:

gem install rails
rails yaffle_guide
cd yaffle_guide
script/generate scaffold bird name:string
rake db:migrate
script/server

Then navigate to http://localhost:3000/birds. Make sure you have a functioning rails app before continuing.

Editor’s note:

The aforementioned instructions will work for sqlite3. For more detailed instructions on how to create a rails app for other databases see the API docs.

1.2. Generate the plugin skeleton

Rails ships with a plugin generator which creates a basic plugin skeleton. Pass the plugin name, either CamelCased or under_scored, as an argument. Pass --with-generator to add an example generator also.

This creates a plugin in vendor/plugins including an init.rb and README as well as standard lib, task, and test directories.

Examples:

./script/generate plugin yaffle
./script/generate plugin yaffle --with-generator

To get more detailed help on the plugin generator, type ./script/generate plugin.

Later on this guide will describe how to work with generators, so go ahead and generate your plugin with the --with-generator option now:

./script/generate plugin yaffle --with-generator

You should see the following output:

create  vendor/plugins/yaffle/lib
create  vendor/plugins/yaffle/tasks
create  vendor/plugins/yaffle/test
create  vendor/plugins/yaffle/README
create  vendor/plugins/yaffle/MIT-LICENSE
create  vendor/plugins/yaffle/Rakefile
create  vendor/plugins/yaffle/init.rb
create  vendor/plugins/yaffle/install.rb
create  vendor/plugins/yaffle/uninstall.rb
create  vendor/plugins/yaffle/lib/yaffle.rb
create  vendor/plugins/yaffle/tasks/yaffle_tasks.rake
create  vendor/plugins/yaffle/test/core_ext_test.rb
create  vendor/plugins/yaffle/generators
create  vendor/plugins/yaffle/generators/yaffle
create  vendor/plugins/yaffle/generators/yaffle/templates
create  vendor/plugins/yaffle/generators/yaffle/yaffle_generator.rb
create  vendor/plugins/yaffle/generators/yaffle/USAGE

1.3. Organize your files

To make it easy to organize your files and to make the plugin more compatible with GemPlugins, start out by altering your file system to look like this:

|-- lib
|   |-- yaffle
|   `-- yaffle.rb
`-- rails
    |
    `-- init.rb

vendor/plugins/yaffle/rails/init.rb

require 'yaffle'

Now you can add any require statements to lib/yaffle.rb and keep init.rb clean.

2. Tests

In this guide you will learn how to test your plugin against multiple different database adapters using Active Record. To setup your plugin to allow for easy testing you’ll need to add 3 files:

A database.yml file with all of your connection strings
A schema.rb file with your table definitions
A test helper method that sets up the database

2.1. Test Setup

vendor/plugins/yaffle/test/database.yml:

sqlite:
  :adapter: sqlite
  :dbfile: vendor/plugins/yaffle/test/yaffle_plugin.sqlite.db

sqlite3:
  :adapter: sqlite3
  :dbfile: vendor/plugins/yaffle/test/yaffle_plugin.sqlite3.db

postgresql:
  :adapter: postgresql
  :username: postgres
  :password: postgres
  :database: yaffle_plugin_test
  :min_messages: ERROR

mysql:
  :adapter: mysql
  :host: localhost
  :username: root
  :password: password
  :database: yaffle_plugin_test

For this guide you’ll need 2 tables/models, Hickwalls and Wickwalls, so add the following:

vendor/plugins/yaffle/test/schema.rb:

ActiveRecord::Schema.define(:version => 0) do
  create_table :hickwalls, :force => true do |t|
    t.string :name
    t.string :last_squawk
    t.datetime :last_squawked_at
  end
  create_table :wickwalls, :force => true do |t|
    t.string :name
    t.string :last_tweet
    t.datetime :last_tweeted_at
  end
  create_table :woodpeckers, :force => true do |t|
    t.string :name
  end
end

vendor/plugins/yaffle/test/test_helper.rb:

ENV['RAILS_ENV'] = 'test'
ENV['RAILS_ROOT'] ||= File.dirname(__FILE__) + '/../../../..'

require 'test/unit'
require File.expand_path(File.join(ENV['RAILS_ROOT'], 'config/environment.rb'))

def load_schema
  config = YAML::load(IO.read(File.dirname(__FILE__) + '/database.yml'))
  ActiveRecord::Base.logger = Logger.new(File.dirname(__FILE__) + "/debug.log")

  db_adapter = ENV['DB']

  # no db passed, try one of these fine config-free DBs before bombing.
  db_adapter ||=
    begin
      require 'rubygems'
      require 'sqlite'
      'sqlite'
    rescue MissingSourceFile
      begin
        require 'sqlite3'
        'sqlite3'
      rescue MissingSourceFile
      end
    end

  if db_adapter.nil?
    raise "No DB Adapter selected. Pass the DB= option to pick one, or install Sqlite or Sqlite3."
  end

  ActiveRecord::Base.establish_connection(config[db_adapter])
  load(File.dirname(__FILE__) + "/schema.rb")
  require File.dirname(__FILE__) + '/../rails/init.rb'
end

Now whenever you write a test that requires the database, you can call load_schema.

2.2. Run the plugin tests

Once you have these files in place, you can write your first test to ensure that your plugin-testing setup is correct. By default rails generates a file in vendor/plugins/yaffle/test/yaffle_test.rb with a sample test. Replace the contents of that file with:

vendor/plugins/yaffle/test/yaffle_test.rb:

require File.dirname(__FILE__) + '/test_helper.rb'

class YaffleTest < Test::Unit::TestCase
  load_schema

  class Hickwall < ActiveRecord::Base
  end

  class Wickwall < ActiveRecord::Base
  end

  def test_schema_has_loaded_correctly
    assert_equal [], Hickwall.all
    assert_equal [], Wickwall.all
  end

end

To run this, go to the plugin directory and run rake:

cd vendor/plugins/yaffle
rake

You should see output like:

/opt/local/bin/ruby -Ilib:lib "/opt/local/lib/ruby/gems/1.8/gems/rake-0.8.3/lib/rake/rake_test_loader.rb" "test/yaffle_test.rb"
-- create_table(:hickwalls, {:force=>true})
   -> 0.0220s
-- create_table(:wickwalls, {:force=>true})
   -> 0.0077s
-- initialize_schema_migrations_table()
   -> 0.0007s
-- assume_migrated_upto_version(0)
   -> 0.0007s
Loaded suite /opt/local/lib/ruby/gems/1.8/gems/rake-0.8.3/lib/rake/rake_test_loader
Started
.
Finished in 0.002236 seconds.

1 test, 1 assertion, 0 failures, 0 errors

By default the setup above runs your tests with sqlite or sqlite3. To run tests with one of the other connection strings specified in database.yml, pass the DB environment variable to rake:

rake DB=sqlite
rake DB=sqlite3
rake DB=mysql
rake DB=postgresql

Now you are ready to test-drive your plugin!

3. Extending core classes

This section will explain how to add a method to String that will be available anywhere in your rails app.

In this example you will add a method to String named to_squawk. To begin, create a new test file with a few assertions:

vendor/plugins/yaffle/test/core_ext_test.rb

require File.dirname(__FILE__) + '/test_helper.rb'

class CoreExtTest < Test::Unit::TestCase
  def test_to_squawk_prepends_the_word_squawk
    assert_equal "squawk! Hello World", "Hello World".to_squawk
  end
end

Navigate to your plugin directory and run rake test:

cd vendor/plugins/yaffle
rake test

The test above should fail with the message:

 1) Error:
test_to_squawk_prepends_the_word_squawk(CoreExtTest):
NoMethodError: undefined method `to_squawk' for "Hello World":String
    ./test/core_ext_test.rb:5:in `test_to_squawk_prepends_the_word_squawk'

Great - now you are ready to start development.

Then in lib/yaffle.rb require lib/core_ext.rb:

vendor/plugins/yaffle/lib/yaffle.rb

require "yaffle/core_ext"

Finally, create the core_ext.rb file and add the to_squawk method:

vendor/plugins/yaffle/lib/yaffle/core_ext.rb

String.class_eval do
  def to_squawk
    "squawk! #{self}".strip
  end
end

To test that your method does what it says it does, run the unit tests with rake from your plugin directory. To see this in action, fire up a console and start squawking:

$ ./script/console
>> "Hello World".to_squawk
=> "squawk! Hello World"

3.1. Working with init.rb

When rails loads plugins it looks for the file named init.rb or rails/init.rb. However, when the plugin is initialized, init.rb is invoked via eval (not require) so it has slightly different behavior.

Under certain circumstances if you reopen classes or modules in init.rb you may inadvertently create a new class, rather than reopening an existing class. A better alternative is to reopen the class in a different file, and require that file from init.rb, as shown above.

If you must reopen a class in init.rb you can use module_eval or class_eval to avoid any issues:

vendor/plugins/yaffle/rails/init.rb

Hash.class_eval do
  def is_a_special_hash?
    true
  end
end

Another way is to explicitly define the top-level module space for all modules and classes, like ::Hash:

vendor/plugins/yaffle/rails/init.rb

class ::Hash
  def is_a_special_hash?
    true
  end
end

4. Add an acts_as method to Active Record

A common pattern in plugins is to add a method called acts_as_something to models. In this case, you want to write a method called acts_as_yaffle that adds a squawk method to your models.

To begin, set up your files so that you have:

vendor/plugins/yaffle/test/acts_as_yaffle_test.rb

require File.dirname(__FILE__) + '/test_helper.rb'

class ActsAsYaffleTest < Test::Unit::TestCase
end

vendor/plugins/yaffle/lib/yaffle.rb

require 'yaffle/acts_as_yaffle'

vendor/plugins/yaffle/lib/yaffle/acts_as_yaffle.rb

module Yaffle
  # your code will go here
end

Note that after requiring acts_as_yaffle you also have to include it into ActiveRecord::Base so that your plugin methods will be available to the rails models.

One of the most common plugin patterns for acts_as_yaffle plugins is to structure your file like so:

vendor/plugins/yaffle/lib/yaffle/acts_as_yaffle.rb

module Yaffle
  def self.included(base)
    base.send :extend, ClassMethods
  end

  module ClassMethods
    # any method placed here will apply to classes, like Hickwall
    def acts_as_something
      send :include, InstanceMethods
    end
  end

  module InstanceMethods
    # any method placed here will apply to instaces, like @hickwall
  end
end

With structure you can easily separate the methods that will be used for the class (like Hickwall.some_method) and the instance (like @hickwell.some_method).

4.1. Add a class method

This plugin will expect that you’ve added a method to your model named last_squawk. However, the plugin users might have already defined a method on their model named last_squawk that they use for something else. This plugin will allow the name to be changed by adding a class method called yaffle_text_field.

To start out, write a failing test that shows the behavior you’d like:

vendor/plugins/yaffle/test/acts_as_yaffle_test.rb

require File.dirname(__FILE__) + '/test_helper.rb'

class Hickwall < ActiveRecord::Base
  acts_as_yaffle
end

class Wickwall < ActiveRecord::Base
  acts_as_yaffle :yaffle_text_field => :last_tweet
end

class ActsAsYaffleTest < Test::Unit::TestCase
  load_schema

  def test_a_hickwalls_yaffle_text_field_should_be_last_squawk
    assert_equal "last_squawk", Hickwall.yaffle_text_field
  end

  def test_a_wickwalls_yaffle_text_field_should_be_last_tweet
    assert_equal "last_tweet", Wickwall.yaffle_text_field
  end
end

To make these tests pass, you could modify your acts_as_yaffle file like so:

vendor/plugins/yaffle/lib/yaffle/acts_as_yaffle.rb

module Yaffle
  def self.included(base)
    base.send :extend, ClassMethods
  end

  module ClassMethods
    def acts_as_yaffle(options = {})
      cattr_accessor :yaffle_text_field
      self.yaffle_text_field = (options[:yaffle_text_field] || :last_squawk).to_s
    end
  end
end

ActiveRecord::Base.send :include, Yaffle

4.2. Add an instance method

This plugin will add a method named squawk to any Active Record objects that call acts_as_yaffle. The squawk method will simply set the value of one of the fields in the database.

To start out, write a failing test that shows the behavior you’d like:

vendor/plugins/yaffle/test/acts_as_yaffle_test.rb

require File.dirname(__FILE__) + '/test_helper.rb'

class Hickwall < ActiveRecord::Base
  acts_as_yaffle
end

class Wickwall < ActiveRecord::Base
  acts_as_yaffle :yaffle_text_field => :last_tweet
end

class ActsAsYaffleTest < Test::Unit::TestCase
  load_schema

  def test_a_hickwalls_yaffle_text_field_should_be_last_squawk
    assert_equal "last_squawk", Hickwall.yaffle_text_field
  end

  def test_a_wickwalls_yaffle_text_field_should_be_last_tweet
    assert_equal "last_tweet", Wickwall.yaffle_text_field
  end

  def test_hickwalls_squawk_should_populate_last_squawk
    hickwall = Hickwall.new
    hickwall.squawk("Hello World")
    assert_equal "squawk! Hello World", hickwall.last_squawk
  end

  def test_wickwalls_squawk_should_populate_last_tweeted_at
    wickwall = Wickwall.new
    wickwall.squawk("Hello World")
    assert_equal "squawk! Hello World", wickwall.last_tweet
  end
end

Run this test to make sure the last two tests fail, then update acts_as_yaffle.rb to look like this:

vendor/plugins/yaffle/lib/yaffle/acts_as_yaffle.rb

module Yaffle
  def self.included(base)
    base.send :extend, ClassMethods
  end

  module ClassMethods
    def acts_as_yaffle(options = {})
      cattr_accessor :yaffle_text_field
      self.yaffle_text_field = (options[:yaffle_text_field] || :last_squawk).to_s
      send :include, InstanceMethods
    end
  end

  module InstanceMethods
    def squawk(string)
      write_attribute(self.class.yaffle_text_field, string.to_squawk)
    end
  end
end

ActiveRecord::Base.send :include, Yaffle

Editor’s note:

The use of write_attribute to write to the field in model is just one example of how a plugin can interact with the model, and will not always be the right method to use. For example, you could also use send("#{self.class.yaffle_text_field}=", string.to_squawk).

5. Models

This section describes how to add a model named Woodpecker to your plugin that will behave the same as a model in your main app. When storing models, controllers, views and helpers in your plugin, it’s customary to keep them in directories that match the rails directories. For this example, create a file structure like this:

vendor/plugins/yaffle/
|-- lib
|   |-- app
|   |   |-- controllers
|   |   |-- helpers
|   |   |-- models
|   |   |   `-- woodpecker.rb
|   |   `-- views
|   |-- yaffle
|   |   |-- acts_as_yaffle.rb
|   |   |-- commands.rb
|   |   `-- core_ext.rb
|   `-- yaffle.rb

As always, start with a test:

vendor/plugins/yaffle/test/woodpecker_test.rb:

require File.dirname(__FILE__) + '/test_helper.rb'

class WoodpeckerTest < Test::Unit::TestCase
  load_schema

  def test_woodpecker
    assert_kind_of Woodpecker, Woodpecker.new
  end
end

This is just a simple test to make sure the class is being loaded correctly. After watching it fail with rake, you can make it pass like so:

vendor/plugins/yaffle/lib/yaffle.rb:

%w{ models }.each do |dir|
  path = File.join(File.dirname(__FILE__), 'app', dir)
  $LOAD_PATH << path
  ActiveSupport::Dependencies.load_paths << path
  ActiveSupport::Dependencies.load_once_paths.delete(path)
end

Adding directories to the load path makes them appear just like files in the the main app directory - except that they are only loaded once, so you have to restart the web server to see the changes in the browser. Removing directories from the load_once_paths allow those changes to picked up as soon as you save the file - without having to restart the web server. This is particularly useful as you develop the plugin.

vendor/plugins/yaffle/lib/app/models/woodpecker.rb:

class Woodpecker < ActiveRecord::Base
end

Finally, add the following to your plugin’s schema.rb:

vendor/plugins/yaffle/test/schema.rb:

create_table :woodpeckers, :force => true do |t|
  t.string :name
end

Now your test should be passing, and you should be able to use the Woodpecker model from within your rails app, and any changes made to it are reflected immediately when running in development mode.

6. Controllers

This section describes how to add a controller named woodpeckers to your plugin that will behave the same as a controller in your main app. This is very similar to adding a model.

You can test your plugin’s controller as you would test any other controller:

vendor/plugins/yaffle/test/woodpeckers_controller_test.rb:

require File.dirname(__FILE__) + '/test_helper.rb'
require 'woodpeckers_controller'
require 'action_controller/test_process'

class WoodpeckersController; def rescue_action(e) raise e end; end

class WoodpeckersControllerTest < Test::Unit::TestCase
  def setup
    @controller = WoodpeckersController.new
    @request = ActionController::TestRequest.new
    @response = ActionController::TestResponse.new

    ActionController::Routing::Routes.draw do |map|
      map.resources :woodpeckers
    end
  end

  def test_index
    get :index
    assert_response :success
  end
end

This is just a simple test to make sure the controller is being loaded correctly. After watching it fail with rake, you can make it pass like so:

vendor/plugins/yaffle/lib/yaffle.rb:

%w{ models controllers }.each do |dir|
  path = File.join(File.dirname(__FILE__), 'app', dir)
  $LOAD_PATH << path
  ActiveSupport::Dependencies.load_paths << path
  ActiveSupport::Dependencies.load_once_paths.delete(path)
end

vendor/plugins/yaffle/lib/app/controllers/woodpeckers_controller.rb:

class WoodpeckersController < ActionController::Base

  def index
    render :text => "Squawk!"
  end

end

Now your test should be passing, and you should be able to use the Woodpeckers controller in your app. If you add a route for the woodpeckers controller you can start up your server and go to http://localhost:3000/woodpeckers to see your controller in action.

7. Helpers

This section describes how to add a helper named WoodpeckersHelper to your plugin that will behave the same as a helper in your main app. This is very similar to adding a model and a controller.

You can test your plugin’s helper as you would test any other helper:

vendor/plugins/yaffle/test/woodpeckers_helper_test.rb

require File.dirname(__FILE__) + '/test_helper.rb'
include WoodpeckersHelper

class WoodpeckersHelperTest < Test::Unit::TestCase
  def test_tweet
    assert_equal "Tweet! Hello", tweet("Hello")
  end
end

This is just a simple test to make sure the helper is being loaded correctly. After watching it fail with rake, you can make it pass like so:

vendor/plugins/yaffle/lib/yaffle.rb:

%w{ models controllers helpers }.each do |dir|
  path = File.join(File.dirname(__FILE__), 'app', dir)
  $LOAD_PATH << path
  ActiveSupport::Dependencies.load_paths << path
  ActiveSupport::Dependencies.load_once_paths.delete(path)
end

vendor/plugins/yaffle/lib/app/helpers/woodpeckers_helper.rb:

module WoodpeckersHelper

  def tweet(text)
    "Tweet! #{text}"
  end

end

Now your test should be passing, and you should be able to use the Woodpeckers helper in your app.

8. Routes

In a standard routes.rb file you use routes like map.connect or map.resources. You can add your own custom routes from a plugin. This section will describe how to add a custom method called that can be called with map.yaffles.

Testing routes from plugins is slightly different from testing routes in a standard rails app. To begin, add a test like this:

vendor/plugins/yaffle/test/routing_test.rb

require "#{File.dirname(__FILE__)}/test_helper"

class RoutingTest < Test::Unit::TestCase

  def setup
    ActionController::Routing::Routes.draw do |map|
      map.yaffles
    end
  end

  def test_yaffles_route
    assert_recognition :get, "/yaffles", :controller => "yaffles_controller", :action => "index"
  end

  private

    def assert_recognition(method, path, options)
      result = ActionController::Routing::Routes.recognize_path(path, :method => method)
      assert_equal options, result
    end
end

Once you see the tests fail by running rake, you can make them pass with:

vendor/plugins/yaffle/lib/yaffle.rb

require "yaffle/routing"

vendor/plugins/yaffle/lib/yaffle/routing.rb

module Yaffle #:nodoc:
  module Routing #:nodoc:
    module MapperExtensions
      def yaffles
        @set.add_route("/yaffles", {:controller => "yaffles_controller", :action => "index"})
      end
    end
  end
end

ActionController::Routing::RouteSet::Mapper.send :include, Yaffle::Routing::MapperExtensions

config/routes.rb

ActionController::Routing::Routes.draw do |map|
  map.yaffles
end

You can also see if your routes work by running rake routes from your app directory.

9. Generators

Many plugins ship with generators. When you created the plugin above, you specified the --with-generator option, so you already have the generator stubs in vendor/plugins/yaffle/generators/yaffle.

Building generators is a complex topic unto itself and this section will cover one small aspect of generators: generating a simple text file.

9.1. Testing generators

Many rails plugin authors do not test their generators, however testing generators is quite simple. A typical generator test does the following:

Creates a new fake rails root directory that will serve as destination
Runs the generator
Asserts that the correct files were generated
Removes the fake rails root

This section will describe how to create a simple generator that adds a file. For the generator in this section, the test could look something like this:

vendor/plugins/yaffle/test/definition_generator_test.rb

require File.dirname(__FILE__) + '/test_helper.rb'
require 'rails_generator'
require 'rails_generator/scripts/generate'

class DefinitionGeneratorTest < Test::Unit::TestCase

  def setup
    FileUtils.mkdir_p(fake_rails_root)
    @original_files = file_list
  end

  def teardown
    FileUtils.rm_r(fake_rails_root)
  end

  def test_generates_correct_file_name
    Rails::Generator::Scripts::Generate.new.run(["yaffle_definition"], :destination => fake_rails_root)
    new_file = (file_list - @original_files).first
    assert_equal "definition.txt", File.basename(new_file)
  end

  private

    def fake_rails_root
      File.join(File.dirname(__FILE__), 'rails_root')
    end

    def file_list
      Dir.glob(File.join(fake_rails_root, "*"))
    end

end

You can run rake from the plugin directory to see this fail. Unless you are doing more advanced generator commands it typically suffices to just test the Generate script, and trust that rails will handle the Destroy and Update commands for you.

To make it pass, create the generator:

vendor/plugins/yaffle/generators/yaffle_definition/yaffle_definition_generator.rb

class YaffleDefinitionGenerator < Rails::Generator::Base
  def manifest
    record do |m|
      m.file "definition.txt", "definition.txt"
    end
  end
end

9.2. The USAGE file

If you plan to distribute your plugin, developers will expect at least a minimum of documentation. You can add simple documentation to the generator by updating the USAGE file.

Rails ships with several built-in generators. You can see all of the generators available to you by typing the following at the command line:

./script/generate

You should see something like this:

Installed Generators
  Plugins (vendor/plugins): yaffle_definition
  Builtin: controller, integration_test, mailer, migration, model, observer, plugin, resource, scaffold, session_migration

When you run script/generate yaffle_definition -h you should see the contents of your vendor/plugins/yaffle/generators/yaffle_definition/USAGE.

For this plugin, update the USAGE file could look like this:

Description:
    Adds a file with the definition of a Yaffle to the app's main directory

10. Generator Commands

You may have noticed above that you can used one of the built-in rails migration commands migration_template. If your plugin needs to add and remove lines of text from existing files you will need to write your own generator methods.

This section describes how you you can create your own commands to add and remove a line of text from config/routes.rb.

To start, add the following test method:

vendor/plugins/yaffle/test/route_generator_test.rb

require File.dirname(__FILE__) + '/test_helper.rb'
require 'rails_generator'
require 'rails_generator/scripts/generate'
require 'rails_generator/scripts/destroy'

class RouteGeneratorTest < Test::Unit::TestCase

  def setup
    FileUtils.mkdir_p(File.join(fake_rails_root, "config"))
  end

  def teardown
    FileUtils.rm_r(fake_rails_root)
  end

  def test_generates_route
    content = <<-END
      ActionController::Routing::Routes.draw do |map|
        map.connect ':controller/:action/:id'
        map.connect ':controller/:action/:id.:format'
      end
    END
    File.open(routes_path, 'wb') {|f| f.write(content) }

    Rails::Generator::Scripts::Generate.new.run(["yaffle_route"], :destination => fake_rails_root)
    assert_match /map\.yaffles/, File.read(routes_path)
  end

  def test_destroys_route
    content = <<-END
      ActionController::Routing::Routes.draw do |map|
        map.yaffles
        map.connect ':controller/:action/:id'
        map.connect ':controller/:action/:id.:format'
      end
    END
    File.open(routes_path, 'wb') {|f| f.write(content) }

    Rails::Generator::Scripts::Destroy.new.run(["yaffle_route"], :destination => fake_rails_root)
    assert_no_match /map\.yaffles/, File.read(routes_path)
  end

  private

    def fake_rails_root
      File.join(File.dirname(__FILE__), "rails_root")
    end

    def routes_path
      File.join(fake_rails_root, "config", "routes.rb")
    end

end

Run rake to watch the test fail, then make the test pass add the following:

vendor/plugins/yaffle/lib/yaffle.rb

require "yaffle/commands"

vendor/plugins/yaffle/lib/yaffle/commands.rb

require 'rails_generator'
require 'rails_generator/commands'

module Yaffle #:nodoc:
  module Generator #:nodoc:
    module Commands #:nodoc:
      module Create
        def yaffle_route
          logger.route "map.yaffle"
          look_for = 'ActionController::Routing::Routes.draw do |map|'
          unless options[:pretend]
            gsub_file('config/routes.rb', /(#{Regexp.escape(look_for)})/mi){|match| "#{match}\n  map.yaffles\n"}
          end
        end
      end

      module Destroy
        def yaffle_route
          logger.route "map.yaffle"
          gsub_file 'config/routes.rb', /\n.+?map\.yaffles/mi, ''
        end
      end

      module List
        def yaffle_route
        end
      end

      module Update
        def yaffle_route
        end
      end
    end
  end
end

Rails::Generator::Commands::Create.send   :include,  Yaffle::Generator::Commands::Create
Rails::Generator::Commands::Destroy.send  :include,  Yaffle::Generator::Commands::Destroy
Rails::Generator::Commands::List.send     :include,  Yaffle::Generator::Commands::List
Rails::Generator::Commands::Update.send   :include,  Yaffle::Generator::Commands::Update

vendor/plugins/yaffle/generators/yaffle_route/yaffle_route_generator.rb

class YaffleRouteGenerator < Rails::Generator::Base
  def manifest
    record do |m|
      m.yaffle_route
    end
  end
end

To see this work, type:

./script/generate yaffle_route
./script/destroy yaffle_route

Editor’s note:

If you haven’t set up the custom route from above, script/destroy will fail and you’ll have to remove it manually.

11. Migrations

If your plugin requires changes to the app’s database you will likely want to somehow add migrations. Rails does not include any built-in support for calling migrations from plugins, but you can still make it easy for developers to call migrations from plugins.

If you have a very simple needs, like creating a table that will always have the same name and columns, then you can use a more simple solution, like creating a custom rake task or method. If your migration needs user input to supply table names or other options, you probably want to opt for generating a migration.

Let’s say you have the following migration in your plugin:

vendor/plugins/yaffle/lib/db/migrate/20081116181115_create_birdhouses.rb:

class CreateBirdhouses < ActiveRecord::Migration
  def self.up
    create_table :birdhouses, :force => true do |t|
      t.string :name
      t.timestamps
    end
  end

  def self.down
    drop_table :birdhouses
  end
end

Here are a few possibilities for how to allow developers to use your plugin migrations:

11.1. Create a custom rake task

vendor/plugins/yaffle/tasks/yaffle_tasks.rake:

namespace :db do
  namespace :migrate do
    desc "Migrate the database through scripts in vendor/plugins/yaffle/lib/db/migrate and update db/schema.rb by invoking db:schema:dump. Target specific version with VERSION=x. Turn off output with VERBOSE=false."
    task :yaffle => :environment do
      ActiveRecord::Migration.verbose = ENV["VERBOSE"] ? ENV["VERBOSE"] == "true" : true
      ActiveRecord::Migrator.migrate("vendor/plugins/yaffle/lib/db/migrate/", ENV["VERSION"] ? ENV["VERSION"].to_i : nil)
      Rake::Task["db:schema:dump"].invoke if ActiveRecord::Base.schema_format == :ruby
    end
  end
end

11.2. Call migrations directly

vendor/plugins/yaffle/lib/yaffle.rb:

Dir.glob(File.join(File.dirname(__FILE__), "db", "migrate", "*")).each do |file|
  require file
end

db/migrate/20081116181115_create_birdhouses.rb:

class CreateBirdhouses < ActiveRecord::Migration
  def self.up
    Yaffle::CreateBirdhouses.up
  end

  def self.down
    Yaffle::CreateBirdhouses.down
  end
end

Editor’s note:

several plugin frameworks such as Desert and Engines provide more advanced plugin functionality.

11.3. Generate migrations

Generating migrations has several advantages over other methods. Namely, you can allow other developers to more easily customize the migration. The flow looks like this:

call your script/generate script and pass in whatever options they need
examine the generated migration, adding/removing columns or other options as necessary

This example will demonstrate how to use one of the built-in generator methods named migration_template to create a migration file. Extending the rails migration generator requires a somewhat intimate knowledge of the migration generator internals, so it’s best to write a test first:

vendor/plugins/yaffle/test/yaffle_migration_generator_test.rb

require File.dirname(__FILE__) + '/test_helper.rb'
require 'rails_generator'
require 'rails_generator/scripts/generate'

class MigrationGeneratorTest < Test::Unit::TestCase

  def setup
    FileUtils.mkdir_p(fake_rails_root)
    @original_files = file_list
  end

  def teardown
    ActiveRecord::Base.pluralize_table_names = true
    FileUtils.rm_r(fake_rails_root)
  end

  def test_generates_correct_file_name
    Rails::Generator::Scripts::Generate.new.run(["yaffle_migration", "some_name_nobody_is_likely_to_ever_use_in_a_real_migration"], :destination => fake_rails_root)
    new_file = (file_list - @original_files).first
    assert_match /add_yaffle_fields_to_some_name_nobody_is_likely_to_ever_use_in_a_real_migrations/, new_file
    assert_match /add_column :some_name_nobody_is_likely_to_ever_use_in_a_real_migrations do |t|/, File.read(new_file)
  end

  def test_pluralizes_properly
    ActiveRecord::Base.pluralize_table_names = false
    Rails::Generator::Scripts::Generate.new.run(["yaffle_migration", "some_name_nobody_is_likely_to_ever_use_in_a_real_migration"], :destination => fake_rails_root)
    new_file = (file_list - @original_files).first
    assert_match /add_yaffle_fields_to_some_name_nobody_is_likely_to_ever_use_in_a_real_migration/, new_file
    assert_match /add_column :some_name_nobody_is_likely_to_ever_use_in_a_real_migration do |t|/, File.read(new_file)
  end

  private
    def fake_rails_root
      File.join(File.dirname(__FILE__), 'rails_root')
    end

    def file_list
      Dir.glob(File.join(fake_rails_root, "db", "migrate", "*"))
    end

end

Editor’s note:

the migration generator checks to see if a migation already exists, and it’s hard-coded to check the db/migrate directory. As a result, if your test tries to generate a migration that already exists in the app, it will fail. The easy workaround is to make sure that the name you generate in your test is very unlikely to actually appear in the app.

After running the test with rake you can make it pass with:

vendor/plugins/yaffle/generators/yaffle_migration/yaffle_migration_generator.rb

class YaffleMigrationGenerator < Rails::Generator::NamedBase
  def manifest
    record do |m|
      m.migration_template 'migration:migration.rb', "db/migrate", {:assigns => yaffle_local_assigns,
        :migration_file_name => "add_yaffle_fields_to_#{custom_file_name}"
      }
    end
  end

  private
    def custom_file_name
      custom_name = class_name.underscore.downcase
      custom_name = custom_name.pluralize if ActiveRecord::Base.pluralize_table_names
      custom_name
    end

    def yaffle_local_assigns
      returning(assigns = {}) do
        assigns[:migration_action] = "add"
        assigns[:class_name] = "add_yaffle_fields_to_#{custom_file_name}"
        assigns[:table_name] = custom_file_name
        assigns[:attributes] = [Rails::Generator::GeneratedAttribute.new("last_squawk", "string")]
      end
    end
end

The generator creates a new file in db/migrate with a timestamp and an add_column statement. It reuses the built in rails migration_template method, and reuses the built-in rails migration template.

It’s courteous to check to see if table names are being pluralized whenever you create a generator that needs to be aware of table names. This way people using your generator won’t have to manually change the generated files if they’ve turned pluralization off.

To run the generator, type the following at the command line:

./script/generate yaffle_migration bird

and you will see a new file:

db/migrate/20080529225649_add_yaffle_fields_to_birds.rb

class AddYaffleFieldsToBirds < ActiveRecord::Migration
  def self.up
    add_column :birds, :last_squawk, :string
  end

  def self.down
    remove_column :birds, :last_squawk
  end
end

12. Rake tasks

When you created the plugin with the built-in rails generator, it generated a rake file for you in vendor/plugins/yaffle/tasks/yaffle_tasks.rake. Any rake task you add here will be available to the app.

Many plugin authors put all of their rake tasks into a common namespace that is the same as the plugin, like so:

vendor/plugins/yaffle/tasks/yaffle_tasks.rake

namespace :yaffle do
  desc "Prints out the word 'Yaffle'"
  task :squawk => :environment do
    puts "squawk!"
  end
end

When you run rake -T from your plugin you will see:

yaffle:squawk             # Prints out the word 'Yaffle'

You can add as many files as you want in the tasks directory, and if they end in .rake Rails will pick them up.

Note that tasks from vendor/plugins/yaffle/Rakefile are not available to the main app.

13. PluginGems

Turning your rails plugin into a gem is a simple and straightforward task. This section will cover how to turn your plugin into a gem. It will not cover how to distribute that gem.

Historically rails plugins loaded the plugin’s init.rb file. In fact some plugins contain all of their code in that one file. To be compatible with plugins, init.rb was moved to rails/init.rb.

It’s common practice to put any developer-centric rake tasks (such as tests, rdoc and gem package tasks) in Rakefile. A rake task that packages the gem might look like this:

vendor/plugins/yaffle/Rakefile:

PKG_FILES = FileList[
  '[a-zA-Z]*',
  'generators/**/*',
  'lib/**/*',
  'rails/**/*',
  'tasks/**/*',
  'test/**/*'
]

spec = Gem::Specification.new do |s|
  s.name = "yaffle"
  s.version = "0.0.1"
  s.author = "Gleeful Yaffler"
  s.email = "yaffle@example.com"
  s.homepage = "http://yafflers.example.com/"
  s.platform = Gem::Platform::RUBY
  s.summary = "Sharing Yaffle Goodness"
  s.files = PKG_FILES.to_a
  s.require_path = "lib"
  s.has_rdoc = false
  s.extra_rdoc_files = ["README"]
end

desc 'Turn this plugin into a gem.'
Rake::GemPackageTask.new(spec) do |pkg|
  pkg.gem_spec = spec
end

To build and install the gem locally, run the following commands:

cd vendor/plugins/yaffle
rake gem
sudo gem install pkg/yaffle-0.0.1.gem

To test this, create a new rails app, add config.gem "yaffle" to environment.rb and all of your plugin’s functionality will be available to you.

14. RDoc Documentation

Once your plugin is stable and you are ready to deploy do everyone else a favor and document it! Luckily, writing documentation for your plugin is easy.

The first step is to update the README file with detailed information about how to use your plugin. A few key things to include are:

Your name
How to install
How to add the functionality to the app (several examples of common use cases)
Warning, gotchas or tips that might help save users time

Once your README is solid, go through and add rdoc comments to all of the methods that developers will use. It’s also customary to add #:nodoc: comments to those parts of the code that are not part of the public api.

Once your comments are good to go, navigate to your plugin directory and run:

rake rdoc

15. Appendix

If you prefer to use RSpec instead of Test::Unit, you may be interested in the RSpec Plugin Generator.

15.1. References

15.2. Contents of lib/yaffle.rb

vendor/plugins/yaffle/lib/yaffle.rb:

require "yaffle/core_ext"
require "yaffle/acts_as_yaffle"
require "yaffle/commands"
require "yaffle/routing"

%w{ models controllers helpers }.each do |dir|
  path = File.join(File.dirname(__FILE__), 'app', dir)
  $LOAD_PATH << path
  ActiveSupport::Dependencies.load_paths << path
  ActiveSupport::Dependencies.load_once_paths.delete(path)
end

# optionally:
# Dir.glob(File.join(File.dirname(__FILE__), "db", "migrate", "*")).each do |file|
#   require file
# end

15.3. Final plugin directory structure

The final plugin should have a directory structure that looks something like this:

|-- MIT-LICENSE
|-- README
|-- Rakefile
|-- generators
|   |-- yaffle_definition
|   |   |-- USAGE
|   |   |-- templates
|   |   |   `-- definition.txt
|   |   `-- yaffle_definition_generator.rb
|   |-- yaffle_migration
|   |   |-- USAGE
|   |   |-- templates
|   |   `-- yaffle_migration_generator.rb
|   `-- yaffle_route
|       |-- USAGE
|       |-- templates
|       `-- yaffle_route_generator.rb
|-- install.rb
|-- lib
|   |-- app
|   |   |-- controllers
|   |   |   `-- woodpeckers_controller.rb
|   |   |-- helpers
|   |   |   `-- woodpeckers_helper.rb
|   |   `-- models
|   |       `-- woodpecker.rb
|   |-- db
|   |   `-- migrate
|   |       `-- 20081116181115_create_birdhouses.rb
|   |-- yaffle
|   |   |-- acts_as_yaffle.rb
|   |   |-- commands.rb
|   |   |-- core_ext.rb
|   |   `-- routing.rb
|   `-- yaffle.rb
|-- pkg
|   `-- yaffle-0.0.1.gem
|-- rails
|   `-- init.rb
|-- tasks
|   `-- yaffle_tasks.rake
|-- test
|   |-- acts_as_yaffle_test.rb
|   |-- core_ext_test.rb
|   |-- database.yml
|   |-- debug.log
|   |-- definition_generator_test.rb
|   |-- migration_generator_test.rb
|   |-- route_generator_test.rb
|   |-- routes_test.rb
|   |-- schema.rb
|   |-- test_helper.rb
|   |-- woodpecker_test.rb
|   |-- woodpeckers_controller_test.rb
|   |-- wookpeckers_helper_test.rb
|   |-- yaffle_plugin.sqlite3.db
|   `-- yaffle_test.rb
`-- uninstall.rb

Ruby on Rails

Sustainable productivity for web-application development

Chapters

The Basics of Creating Rails Plugins

1. Setup

1.1. Create the basic app

1.2. Generate the plugin skeleton

1.3. Organize your files

2. Tests

2.1. Test Setup

2.2. Run the plugin tests

3. Extending core classes

3.1. Working with init.rb

4. Add an acts_as method to Active Record

4.1. Add a class method

4.2. Add an instance method

5. Models

6. Controllers

7. Helpers

8. Routes

9. Generators

9.1. Testing generators

9.2. The USAGE file

10. Generator Commands

11. Migrations

11.1. Create a custom rake task

11.2. Call migrations directly

11.3. Generate migrations

12. Rake tasks

13. PluginGems

14. RDoc Documentation

15. Appendix

15.1. References

15.2. Contents of lib/yaffle.rb

15.3. Final plugin directory structure