Tutorial: Creating plugins in Rails

May 29, 2008 rails tutorials

This blog post was written in 2008. Information and links in this post may be outdated.
You can see the most recent version of this post at http://guides.rubyonrails.org/plugins.html

There's a new effort underway to add tutorials to the Rails API documentation. Here's the first draft of a tutorial I just wrote describing how to create plugins.

Creating Plugin Basics

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.

In this tutorial you will learn how to create a plugin that includes:

Core Extensions - extending String:

# Anywhere
"hello".squawk # => "squawk! hello! squawk!"

An acts_as_yaffle method for Active Record models that adds a "squawk" method:

class Hickwall < ActiveRecord::Base
  acts_as_yaffle :yaffle_text_field => :last_sang_at
end

Hickwall.new.squawk("Hello World")

A view helper that will print out squawking info:

squawk_info_for(@hickwall)

A generator that creates a migration to add squawk columns to a model:

script/generate yaffle hickwall

A custom generator command:

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

A custom route method:

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

In addition you'll learn how to:

  • test your plugins
  • work with init.rb, how to store model, views, controllers, helpers and even other plugins in your plugins
  • create documentation for your plugin.

Create the basic app

In this tutorial we will create a basic rails application with 1 resource: bird. Start out by building the basic rails app:

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

rails plugin_demo
cd plugin_demo
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.

Create the plugin

The built-in Rails plugin generator stubs out a new plugin. 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 BrowserFilters
./script/generate plugin BrowserFilters --with-generator

Later in the plugin we will create a generator, so go ahead and add 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

For this plugin you won't need the file vendor/plugins/yaffle/lib/yaffle.rb so you can delete that.

rm vendor/plugins/yaffle/lib/yaffle.rb

Editor's note: many plugin authors prefer to keep this file, and add all of the require statements in it. That way, they only line in init.rb would be require "yaffle" If you are developing a plugin that has a lot of files in the lib directory, you may want to create a subdirectory like lib/yaffle and store your files in there. That way your init.rb file stays clean

Testing Setup

Testing plugins that use the entire Rails stack can be complex, and the generator doesn't offer any help. In this tutorial you will learn how to test your plugin against multiple different adapters using ActiveRecord. This tutorial 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 that sets up the database before your tests

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

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

sqlite:
  :adapter: sqlite
  :dbfile: yaffle_plugin.sqlite.db
sqlite3:
  :adapter: sqlite3
  :dbfile: yaffle_plugin.sqlite3.db
postgresql:
  :adapter: postgresql
  :username: postgres
  :password: postgres
  :database: yaffle_plugin_test
  :min_messages: ERROR
mysql:
  :adapter: mysql
  :host: localhost
  :username: rails
  :password:
  :database: yaffle_plugin_test

# File: vendor/plugins/yaffle/test/test_helper.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
end

# File: 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'))

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__) + '/../init.rb'

class Hickwall < ActiveRecord::Base
  acts_as_yaffle
end

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

Update a core class: Adding "to_squawk" to String

To update a core class you will have to:

  • Write tests for the desired functionality
  • Create a file for the code you wish to use
  • Require that file from your init.rb

Most plugins store their code classes in the plugin's lib directory. When you add a file to the lib directory, you must also require that file from init.rb. The file you are going to add for this tutorial is lib/core_ext.rb

First, you need to write the tests. Testing plugins is very similar to testing rails apps. The generated test file should look something like this:

# File: vendor/plugins/yaffle/test/core_ext_test.rb

require 'test/unit'

class CoreExtTest < Test::Unit::TestCase
  # Replace this with your real tests.
  def test_this_plugin
    flunk
  end
end

Start off by removing the default test, and adding a require statement for your test helper.

# File: vendor/plugins/yaffle/test/core_ext_test.rb

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

class CoreExtTest < Test::Unit::TestCase
end

Navigate to your plugin directory and run rake test

cd vendor/plugins/yaffle
rake test

Your test should fail with no such file to load -- ./test/../lib/core_ext.rb (LoadError) because we haven't created any file yet. Create the file lib/core_ext.rb and re-run the tests. You should see a different error message:

1.) Failure ...
No tests were specified

Great - now you are ready to start development. The first thing we'll do is to add a method to String called to_squawk which will prefix the string with the word "squawk! ". The test will look something like this:

# File: vendor/plugins/yaffle/init.rb

class CoreExtTest < Test::Unit::TestCase
  def test_string_should_respond_to_squawk
    assert_equal true, "".respond_to?(:to_squawk)
  end
  def test_string_prepend_empty_strings_with_the_word_squawk
    assert_equal "squawk!", "".to_squawk
  end
  def test_string_prepend_non_empty_strings_with_the_word_squawk
    assert_equal "squawk! Hello World", "Hello World".to_squawk
  end
end

# File: vendor/plugins/yaffle/init.rb

require "core_ext"

# File: vendor/plugins/yaffle/lib/core_ext.rb

class String
  # returns the current string, prefixed by "squawk!"
  def to_squawk
    "squawk! #{self}".strip
  end
end

To test that your method does what it says it does, run the unit tests. To make sure your code is picked up by

To test this, fire up a console and start squawking:

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

If that worked, congratulations! You just created your first test-driven plugin that extends a core ruby class.

Adding an acts_as_yaffle method to ActiveRecord models

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 keep things clean, create a new test file called acts_as_yaffle_test.rb in your plugin's test directory and require your test helper.

# File: 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 ActsAsYaffleTest < Test::Unit::TestCase
end

# File: vendor/plugins/lib/acts_as_yaffle.rb

module Yaffle
end

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

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).

Let's add class method named acts_as_yaffle - testing it out first. You already defined the ActiveRecord models in your test helper, so if you run tests now they will fail.

Back in your acts\_as\_yaffle file, update ClassMethods like so:

module ClassMethods
  def acts_as_yaffle(options = {})
    send :include, InstanceMethods
  end
end

Now that test should pass. Since your plugin is going to work with field names, you need to allow people to define the field names, in case there is a naming conflict. You can write a few simple tests for this:

# File: vendor/plugins/yaffle/test/acts_as_yaffle_test.rb

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

class ActsAsYaffleTest < Test::Unit::TestCase
  def test_a_hickwalls_yaffle_text_field_should_be_last_squawk
    assert_equal "last_squawk", Hickwall.yaffle_text_field
  end
  def test_a_hickwalls_yaffle_date_field_should_be_last_squawked_at
    assert_equal "last_squawked_at", Hickwall.yaffle_date_field
  end
  def test_a_wickwalls_yaffle_text_field_should_be_last_tweet
    assert_equal "last_tweet", Wickwall.yaffle_text_field
  end
  def test_a_wickwalls_yaffle_date_field_should_be_last_tweeted_at
    assert_equal "last_tweeted_at", Wickwall.yaffle_date_field
  end
end

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

# File: vendor/plugins/yaffle/lib/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, :yaffle_date_field
      self.yaffle_text_field = (options[:yaffle_text_field] || :last_squawk).to_s
      self.yaffle_date_field = (options[:yaffle_date_field] || :last_squawked_at).to_s
      send :include, InstanceMethods
    end
  end

  module InstanceMethods
  end
end

Now you can add tests for the instance methods, and the instance method itself:

# File: vendor/plugins/yaffle/test/acts_as_yaffle_test.rb

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

class ActsAsYaffleTest < Test::Unit::TestCase

  def test_a_hickwalls_yaffle_text_field_should_be_last_squawk
    assert_equal "last_squawk", Hickwall.yaffle_text_field
  end
  def test_a_hickwalls_yaffle_date_field_should_be_last_squawked_at
    assert_equal "last_squawked_at", Hickwall.yaffle_date_field
  end

  def test_a_wickwalls_yaffle_text_field_should_be_last_squawk
    assert_equal "last_tweet", Wickwall.yaffle_text_field
  end
  def test_a_wickwalls_yaffle_date_field_should_be_last_squawked_at
    assert_equal "last_tweeted_at", Wickwall.yaffle_date_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_hickwalls_squawk_should_populate_last_squawked_at
    hickwall = Hickwall.new
    hickwall.squawk("Hello World")
    assert_equal Date.today, hickwall.last_squawked_at
  end

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

# File: vendor/plugins/yaffle/lib/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, :yaffle_date_field
      self.yaffle_text_field = (options[:yaffle_text_field] || :last_squawk).to_s
      self.yaffle_date_field = (options[:yaffle_date_field] || :last_squawked_at).to_s
      send :include, InstanceMethods
    end
  end

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

Note the use of write_attribute to write to the field in model.

Create a view helper

Creating a view helper is a 3-step process:

  • Add an appropriately named file to the lib directory
  • Require the file and hooks in init.rb
  • Write the tests

First, create the test to define the functionality you want:

# File: vendor/plugins/yaffle/test/view_helpers_test.rb

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

class ViewHelpersTest < Test::Unit::TestCase
  def test_squawk_info_for_should_return_the_text_and_date
    time = Time.now
    hickwall = Hickwall.new
    hickwall.last_squawk = "Hello World"
    hickwall.last_squawked_at = time
    assert_equal "Hello World, #{time.to_s}", squawk_info_for(hickwall)
  end
end

Then add the following statements to init.rb:

# File: vendor/plugins/yaffle/init.rb

require "view_helpers"
ActionView::Base.send :include, YaffleViewHelper

Then add the view helpers file and

# File: vendor/plugins/yaffle/lib/view_helpers.rb

module YaffleViewHelper
  def squawk_info_for(yaffle)
    returning "" do |result|
      result << yaffle.read_attribute(yaffle.class.yaffle_text_field)
      result << ", "
      result << yaffle.read_attribute(yaffle.class.yaffle_date_field).to_s
    end
  end
end

You can also test this in script/console by using the "helper" method:

script/console
>> helper.squawk_info_for(@some_yaffle_instance)

Create a migration generator

When you created the plugin above, you specified the --with-generator option, so you already have the generator stubs in your plugin.

We'll be relying on the built-in rails generate template for this tutorial. Going into the details of generators is beyond the scope of this tutorial.

Type:

script/generate

You should see the line:

Plugins (vendor/plugins): yaffle

When you run script/generate yaffle you should see the contents of your USAGE file. For this plugin, 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

Now you can add code to your generator:

# File: 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")]
        assigns[:attributes] << Rails::Generator::GeneratedAttribute.new("last_squawked_at", "datetime")
      end
    end
end

Note that you need to be aware of whether or not table names are pluralized.

This does a few things:

  • Reuses the built in rails migration_template method
  • Reuses the built-in rails migration template

When you run the generator like

script/generate yaffle bird

You will see a new file:

# File: db/migrate/20080529225649_add_yaffle_fields_to_birds.rb

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

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

Adding custom generator commands

You may have noticed above that you can used one of the built-in rails migration commands m.migration_template. You can create your own commands for these, using the following steps:

  1. Add the require and hook statements to init.rb
  2. Create the commands - creating 3 sets, Create, Destroy, List
  3. Add the method to your generator

Working with the internals of generators is beyond the scope of this tutorial, but here is a basic example:

# File: vendor/plugins/yaffle/init.rb

require "commands"
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

# File: 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
    end
  end
end

# File: vendor/plugins/yaffle/generators/yaffle/templates/definition.txt

Yaffle is a bird

# File: vendor/plugins/yaffle/generators/yaffle/yaffle_generator.rb

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

This example just uses the built-in "file" method, but you could do anything that ruby allows.

Adding Routes

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

# File: 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

# File: vendor/plugins/yaffle/init.rb

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

# File: 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

# File: 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.

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

Working with init.rb

The plugin initializer script init.rb is invoked via eval (not require) so it has slightly different behavior.

If you reopen any classes in init.rb itself your changes will potentially be made to the wrong module. There are 2 ways around this:

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

# File: vendor/plugins/yaffle/init.rb

class ::Hash
  def is_a_special_hash?
    true
  end
end

OR you can use module_eval or class_eval

# File: vendor/plugins/yaffle/init.rb

Hash.class_eval do
  def is_a_special_hash?
    true
  end
end

Storing models, views, helpers, and controllers in your plugins

You can easily store models, views, helpers and controllers in plugins. Just create a folder for each in the lib folder, add them to the load path and remove them from the load once path:

# File: vendor/plugins/yaffle/init.rb

%w{ models controllers helpers }.each do |dir|
  path = File.join(directory, 'lib', dir)
  $LOAD_PATH << path
  Dependencies.load_paths << path
  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.

Adding directories to the load once paths allow those changes to picked up as soon as you save the file - without having to restart the web server.

Storing 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")

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.

Custom Plugin Generators

If you are an RSpec fan, you can install the rspec_plugin_generator, which will generate the spec folder and database for you.

http://github.com/pat-maddox/rspec-plugin-generator/tree/master

References

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