The Basics of Creating Rails Plugins

# Ruby on Rails

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# 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. Preparation

### 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

To begin just change one thing - move init.rb to rails/init.rb.

### 1.3. Setup the plugin for testing

If your plugin interacts with a database, you'll need to setup a database connection. In this guide you will learn how to test your plugin against multiple different database adapters using Active Record. This guide will not cover how to use fixtures in plugin tests.

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

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.

### 1.4. 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!

## 2. Extending core classes

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

- Writing tests for the desired behavior

- Creating and requiring the correct files

### 2.1. Creating the test

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.

### 2.2. Organize your files

A common pattern in rails plugins is to set up the file structure like this:

    |-- lib
    |   |-- yaffle
    |   |   `-- core_ext.rb
    |   `-- yaffle.rb

The first thing we need to to is to require our lib/yaffle.rb file from rails/init.rb:

vendor/plugins/yaffle/rails/init.rb

    require 'yaffle'

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"

### 2.3. Working with init.rb

When rails loads plugins it looks for the file named 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/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/init.rb

    class ::Hash
      def is_a_special_hash?
        true
      end
    end

## 3. Add an `acts_as_yaffle` 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`).

### 3.1. 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

### 3.2. 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)`.

## 4. Create a generator

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: creating a generator that adds a time-stamped migration.

To create a generator you must:

- Add your instructions to the manifest method of the generator

- Add any necessary template files to the templates directory

- Test the generator manually by running various combinations of `script/generate` and `script/destroy`

- Update the USAGE file to add helpful documentation for your generator

### 4.1. 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 forward and backward, making whatever assertions are necessary

- Removes the fake rails root

For the generator in this section, the test could look something like this:

vendor/plugins/yaffle/test/yaffle_generator_test.rb

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

    class GeneratorTest < Test::Unit::TestCase

      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

      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", "bird"], :destination => fake_rails_root)
        new_file = (file_list - @original_files).first
        assert_match /add_yaffle_fields_to_bird/, new_file
      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.

### 4.2. Adding to the manifest

This example will demonstrate how to use one of the built-in generator methods named migration_template to create a migration file. To start, update your generator file to look like this:

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

    class YaffleGenerator < 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
        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.

### 4.3. Manually test the generator

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

    ./script/generate yaffle 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

### 4.4. 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
      Builtin: controller, integration_test, mailer, migration, model, observer, plugin, resource, scaffold, session_migration

When you run `script/generate yaffle` you should see the contents of your vendor/plugins/yaffle/generators/yaffle/USAGE file.

For this plugin, update the USAGE file looks like this:

    Description:
        Creates a migration that adds yaffle squawk fields to the given model

    Example:
        ./script/generate yaffle hickwall

        This will create:
            db/migrate/TIMESTAMP_add_yaffle_fields_to_hickwall

## 5. 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

    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

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

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

    Yaffle: A bird

vendor/plugins/yaffle/lib/yaffle.rb

    require "yaffle/commands"

vendor/plugins/yaffle/lib/commands.rb

    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

Finally, call your new method in the manifest:

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

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

## 6. Add a model

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/yaffle/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:

    ActiveRecord::Schema.define(:version => 0) do
      create_table :woodpeckers, :force => true do |t|
        t.string :name
      end
    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.

## 7. Add a controller

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/yaffle/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
      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.

## 8. Add a helper

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

    ActionView::Base.send :include, WoodpeckersHelper

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.

## 9. Add a Custom Route

Testing routes in plugins can be complex, especially if the controllers are also in the plugin itself. Jamis Buck showed a great example of this in http://weblog.jamisbuck.org/2006/10/26/monkey-patching-rails-extending-routes-2.

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

        # yes, I know about assert_recognizes, but it has proven problematic to
        # use in these tests, since it uses RouteSet#recognize (which actually
        # tries to instantiate the controller) and because it uses an awkward
        # parameter order.
        def assert_recognition(method, path, options)
          result = ActionController::Routing::Routes.recognize_path(path, :method => method)
          assert_equal options, result
        end
    end

vendor/plugins/yaffle/init.rb

    require "routing"
    ActionController::Routing::RouteSet::Mapper.send :include, Yaffle::Routing::MapperExtensions

vendor/plugins/yaffle/lib/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

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.

## 10. Odds and ends

### 10.1. Generate RDoc Documentation

Once your plugin is stable, the tests pass on all database 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.

Before you generate your documentation, be sure to go through and add nodoc comments to those modules and methods that are not important to your users.

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

    rake rdoc

### 10.2. Write custom Rake tasks in your plugin

When you created the plugin with the built-in rails generator, it generated a rake file for you in vendor/plugins/yaffle/tasks/yaffle.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.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.

### 10.3. Store plugins in alternate locations

You can store plugins wherever you want - you just have to add those plugins to the plugins path in environment.rb.

Since the plugin is only loaded after the plugin paths are defined, you can't redefine this in your plugins - but it may be good to now.

You can even store plugins inside of other plugins for complete plugin madness!

    config.plugin_paths << File.join(RAILS_ROOT,"vendor","plugins","yaffle","lib","plugins")

### 10.4. Create your own Plugin Loaders and Plugin Locators

If the built-in plugin behavior is inadequate, you can change almost every aspect of the location and loading process. You can write your own plugin locators and plugin loaders, but that's beyond the scope of this tutorial.

### 10.5. Use Custom Plugin Generators

If you are an RSpec fan, you can install the `rspec_plugin_generator` gem, which will generate the spec folder and database for you. See http://github.com/pat-maddox/rspec-plugin-generator/tree/master.

## 11. Appendix

### 11.1. References

- http://nubyonrails.com/articles/the-complete-guide-to-rails-plugins-part-i

- http://nubyonrails.com/articles/2006/05/09/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

### 11.2. Final plugin directory structure

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

      |-- MIT-LICENSE
      |-- README
      |-- Rakefile
      |-- generators
      |   `-- yaffle
      |       |-- USAGE
      |       |-- templates
      |       |   `-- definition.txt
      |       `-- yaffle_generator.rb
      |-- init.rb
      |-- install.rb
      |-- lib
      |   |-- acts_as_yaffle.rb
      |   |-- commands.rb
      |   |-- core_ext.rb
      |   |-- routing.rb
      |   `-- view_helpers.rb
      |-- tasks
      |   `-- yaffle_tasks.rake
      |-- test
      |   |-- acts_as_yaffle_test.rb
      |   |-- core_ext_test.rb
      |   |-- database.yml
      |   |-- debug.log
      |   |-- routing_test.rb
      |   |-- schema.rb
      |   |-- test_helper.rb
      |   `-- view_helpers_test.rb
      |-- uninstall.rb
      `-- yaffle_plugin.sqlite3.db