h2. 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.

endprologue.

h3. Setup

h4. Create the Basic Application

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

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

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

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.


h4. 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:
<pre>
./script/generate plugin yaffle
./script/generate plugin yaffle --with-generator
</pre>

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:

<pre>
./script/generate plugin yaffle --with-generator
</pre>

You should see the following output:

<pre>
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
</pre>

h4. 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:

<pre>
|-- lib
|   |-- yaffle
|   `-- yaffle.rb
`-- rails
    |
    `-- init.rb
</pre>

*vendor/plugins/yaffle/rails/init.rb*

<ruby>
require 'yaffle'
</ruby>

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

h3. 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

h4. Test Setup

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

<pre>
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
</pre>

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

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

<ruby>
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
</ruby>

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

<ruby>
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
</ruby>

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

h4. 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:*

<ruby>
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
</ruby>

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

<pre>
cd vendor/plugins/yaffle
rake
</pre>

You should see output like:

<shell>
/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
</shell>

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:

<shell>
rake DB=sqlite
rake DB=sqlite3
rake DB=mysql
rake DB=postgresql
</shell>

Now you are ready to test-drive your plugin!

h3. 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*

<ruby>
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
</ruby>

Navigate to your plugin directory and run +rake test+:

<shell>
cd vendor/plugins/yaffle
rake test
</shell>

The test above should fail with the message:

<shell>
 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'
</shell>

Great - now you are ready to start development.

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

* *vendor/plugins/yaffle/lib/yaffle.rb*

<ruby>
require "yaffle/core_ext"
</ruby>

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

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

<ruby>
String.class_eval do
  def to_squawk
    "squawk! #{self}".strip
  end
end
</ruby>

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:

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

h4. 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*

<ruby>
Hash.class_eval do
  def is_a_special_hash?
    true
  end
end
</ruby>

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

* *vendor/plugins/yaffle/rails/init.rb*

<ruby>
class ::Hash
  def is_a_special_hash?
    true
  end
end
</ruby>

h3. 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*

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

class ActsAsYaffleTest < Test::Unit::TestCase
end
</ruby>

* *vendor/plugins/yaffle/lib/yaffle.rb*

<ruby>
require 'yaffle/acts_as_yaffle'
</ruby>

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

<ruby>
module Yaffle
  # your code will go here
end
</ruby>

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*

<ruby>
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
</ruby>

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+).

h4. 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*

<ruby>
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
</ruby>

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

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

<ruby>
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
</ruby>

h4. 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*

<ruby>
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
</ruby>

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*

<ruby>
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
</ruby>

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)+.

h3. 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:

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

As always, start with a test:

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

<ruby>
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
</ruby>

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:*

<ruby>
%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
</ruby>

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:*

<ruby>
class Woodpecker < ActiveRecord::Base
end
</ruby>

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

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

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

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.

h3. 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:*

<ruby>
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
</ruby>

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:*

<ruby>
%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
</ruby>

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

<ruby>
class WoodpeckersController < ActionController::Base

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

end
</ruby>

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.

h3. 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*

<ruby>
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
</ruby>

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:*

<ruby>
%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
</ruby>

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

<ruby>
module WoodpeckersHelper

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

end
</ruby>

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

h3. 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*

<ruby>
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
</ruby>

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

* *vendor/plugins/yaffle/lib/yaffle.rb*

<ruby>
require "yaffle/routing"
</ruby>

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

<ruby>
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
</ruby>

* *config/routes.rb*

<ruby>
ActionController::Routing::Routes.draw do |map|
  map.yaffles
end
</ruby>

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

h3. 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.

h4. 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*

<ruby>
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
</ruby>

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*

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

h4. 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:

<shell>
./script/generate
</shell>

You should see something like this:

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

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:

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

h3. Add a Custom Generator Command

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 'routes.rb'.  This example creates a very simple method that adds or removes a text file.

To start, add the following test method:

* *vendor/plugins/yaffle/test/generator_test.rb*

<ruby>
def test_generates_definition
  Rails::Generator::Scripts::Generate.new.run(["yaffle", "bird"], :destination => fake_rails_root)
  definition = File.read(File.join(fake_rails_root, "definition.txt"))
  assert_match /Yaffle\:/, definition
end
</ruby>

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

* *vendor/plugins/yaffle/generators/yaffle/templates/definition.txt*

<shell>
Yaffle: A bird
</shell>

* *vendor/plugins/yaffle/lib/yaffle.rb*

<ruby>
require "yaffle/commands"
</ruby>

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

<ruby>
require 'rails_generator'
require 'rails_generator/commands'

module Yaffle #:nodoc:
  module Generator #:nodoc:
    module Commands #:nodoc:
      module Create
        def yaffle_definition
          file("definition.txt", "definition.txt")
        end
      end

      module Destroy
        def yaffle_definition
          file("definition.txt", "definition.txt")
        end
      end

      module List
        def yaffle_definition
          file("definition.txt", "definition.txt")
        end
      end

      module Update
        def yaffle_definition
          file("definition.txt", "definition.txt")
        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
</ruby>

Finally, call your new method in the manifest:

* *vendor/plugins/yaffle/generators/yaffle/yaffle_generator.rb*

<ruby>
class YaffleGenerator < Rails::Generator::NamedBase
  def manifest
    m.yaffle_definition
  end
end
</ruby>

h3. 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*

<ruby>
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
</ruby>

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

* *vendor/plugins/yaffle/lib/yaffle.rb*

<ruby>
require "yaffle/commands"
</ruby>

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

<ruby>
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
</ruby>

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

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

To see this work, type:

<shell>
./script/generate yaffle_route
./script/destroy yaffle_route
</shell>

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

h3. 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:*

<ruby>
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
</ruby>

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

h4. Create a Custom Rake Task

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

<ruby>
namespace :db do
  namespace :migrate do
    description = "Migrate the database through scripts in vendor/plugins/yaffle/lib/db/migrate"
    description << "and update db/schema.rb by invoking db:schema:dump."
    description << "Target specific version with VERSION=x. Turn off output with VERBOSE=false."

    desc description
    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
</ruby>

h4. Call Migrations Directly

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

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

* *db/migrate/20081116181115_create_birdhouses.rb:*

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

  def self.down
    Yaffle::CreateBirdhouses.down
  end
end
</ruby>

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

h4. 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*

<ruby>
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
</ruby>

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*

<ruby>
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
</ruby>

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:

<shell>
./script/generate yaffle_migration bird
</shell>

and you will see a new file:

* *db/migrate/20080529225649_add_yaffle_fields_to_birds.rb*

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

  def self.down
    remove_column :birds, :last_squawk
  end
end
</ruby>

h3. 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*

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

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

<shell>
yaffle:squawk             # Prints out the word 'Yaffle'
</shell>

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.

h3. 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:*

<ruby>
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
</ruby>

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

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

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.

h3. 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:

<shell>
rake rdoc
</shell>

h3. Appendix

If you prefer to use RSpec instead of Test::Unit, you may be interested in the "RSpec Plugin Generator":http://github.com/pat-maddox/rspec-plugin-generator/tree/master.

h4. References

* http://nubyonrails.com/articles/the-complete-guide-to-rails-plugins-part-i
* http://nubyonrails.com/articles/the-complete-guide-to-rails-plugins-part-ii
* http://github.com/technoweenie/attachment_fu/tree/master
* http://daddy.platte.name/2007/05/rails-plugins-keep-initrb-thin.html
* http://www.mbleigh.com/2008/6/11/gemplugins-a-brief-introduction-to-the-future-of-rails-plugins
* http://weblog.jamisbuck.org/2006/10/26/monkey-patching-rails-extending-routes-2.

h4. Contents of +lib/yaffle.rb+

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

<ruby>
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
</ruby>

h4. Final Plugin Directory Structure

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

<shell>
|-- 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
</shell>

h3. Changelog

"Lighthouse ticket":http://rails.lighthouseapp.com/projects/16213/tickets/32-update-plugins-guide

* November 17, 2008: Major revision by Jeff Dean
