In this guide you will learn about engines and how they can be used to provide additional functionality to their host applications through a clean and very easy-to-use interface. You will learn the following things in this guide:
- What makes an engine
- How to generate an engine
- Building features for the engine
- Hooking the engine into an application
- Overriding engine functionality in the application
endprologue.
Engines can be considered miniature applications that provide functionality to their host applications. A Rails application is actually just a “supercharged” engine, with the Rails::Application class inheriting from Rails::Engine. Therefore, engines and applications share common functionality but are at the same time two separate beasts. Engines and applications also share a common structure, as you’ll see througout this guide.
Engines are also closely related to plugins where the two share a common lib directory structure and are both generated using the rails plugin new generator.
The engine that will be generated for this guide will be called “blorgh”. The engine will provide blogging functionality to its host applications, allowing for new posts and comments to be created. For now, you will be working solely within the engine itself and in later sections you’ll see how to hook it into an application.
Engines can also be isolated from their host applications. This means that an application is able to have a path provided by a routing helper such as posts_path and use an engine also that provides a path also called posts_path, and the two would not clash. Along with this, controllers, models and table names are also namespaced. You’ll see how to do this later in this guide.
To see demonstrations of other engines, check out Devise, an engine that provides authentication for its parent applications, or Forem, an engine that provides forum functionality.
Finally, engines would not have be possible without the work of James Adam, Piotr Sarnacki, the Rails Core Team, and a number of other people. If you ever meet them, don’t forget to say thanks!
To generate an engine with Rails 3.1, you will need to run the plugin generator and pass it the —mountable option. To generate the beginnings of the “blorgh” engine you will need to run this command in a terminal:
$ rails plugin new blorgh —mountable
The —mountable option tells the plugin generator that you want to create an engine (which is a mountable plugin, hence the option name), creating the basic directory structure of an engine by providing things such as the foundations of an app folder, as well a config/routes.rb file. This generator also provides a file at lib/blorgh/engine.rb which is identical in function to an application’s config/application.rb file.
At the root of the engine’s directory, lives a blorgh.gemspec file. When you include the engine into the application later on, you will do so with this line in a Rails application’s Gemfile:
gem ‘blorgh’, :path => “vendor/engines/blorgh”By specifying it as a gem within the Gemfile, Bundler will load it as such, parsing this blorgh.gemspec file and requiring a file within the lib directory called lib/blorgh.rb. This file requires the blorgh/engine.rb file (located at lib/blorgh/engine.rb) and defines a base module called Blorgh.
require “blorgh/engine”
module Blorgh
end
Within lib/blorgh/engine.rb is the base class for the engine:
module Blorgh
class Engine < Rails::Engine
isolate_namespace Blorgh
end
end
By inheriting from the Rails::Engine class, this engine gains all the functionality it needs, such as being able to serve requests to its controllers.
The isolate_namespace method here deserves special notice. This call is responsible for isolating the controllers, models, routes and other things into their own namespace. Without this, there is a possibility that the engine’s components could “leak” into the application, causing unwanted disruption. It is recommended that this line be left within this file.
Inside the app directory there lives the standard assets, controllers, helpers, mailers, models and views directories that you should be familiar with from an application. The helpers, mailers and models directories are empty and so aren’t described in this section. We’ll look more into models in a future section.
Within the app/assets directory, there is the images, javascripts and stylesheets directories which, again, you should be familiar with due to their similarities of an application. One difference here however is that each directory contains a sub-directory with the engine name. Because this engine is going to be namespaced, its assets should be too.
Within the app/controllers directory there is a blorgh directory and inside that a file called application_controller.rb. This file will provide any common functionality for the controllers of the engine. The blorgh directory is where the other controllers for the engine will go. By placing them within this namespaced directory, you prevent them from possibly clashing with identically-named controllers within other engines or even within the application.
Lastly, the app/views directory contains a layouts folder which contains file at blorgh/application.html.erb which allows you to specify a layout for the engine. If this engine is to be used as a stand-alone engine, then you would add any customization to its layout in this file, rather than the applications app/views/layouts/application.html.erb file.
This directory contains one file, script/rails, which allows you to use the rails sub-commands and generators just like you would within an application. This means that you will very easily be able to generate new controllers and models for this engine.
The test directory is where tests for the engine will go. To test the engine, there is a cut-down version of a Rails application embedded within it at test/dummy. This application will mount the engine in the test/dummy/config/routes.rb file:
Rails.application.routes.draw do
mount Blorgh::Engine => “/blorgh”end
This line mounts the engine at the path of /blorgh, which will make it accessible through the application only at that path. We will look more into mounting an engine after some features have been developed.
Also in the test directory is the test/integration directory, where integration tests for the engine should be placed.
The engine that this guide covers will provide posting and commenting functionality.
The first thing to generate for a blog engine is the Post model and related controller. To quickly generate this, you can use the Rails scaffold generator.
$ rails generate scaffold post title:string text:text
This command will output this information:
invoke active_record
create db/migrate/[timestamp]create_blorgh_posts.rb
create app/models/blorgh/post.rb
invoke test_unit
create test/unit/blorgh/post_test.rb
create test/fixtures/blorgh/posts.yml
route resources :posts
invoke scaffold_controller
create app/controllers/blorgh/posts_controller.rb
invoke erb
create app/views/blorgh/posts
create app/views/blorgh/posts/index.html.erb
create app/views/blorgh/posts/edit.html.erb
create app/views/blorgh/posts/show.html.erb
create app/views/blorgh/posts/new.html.erb
create app/views/blorgh/posts/form.html.erb
invoke test_unit
create test/functional/blorgh/posts_controller_test.rb
invoke helper
create app/helpers/blorgh/posts_helper.rb
invoke test_unit
create test/unit/helpers/blorgh/postshelpertest.rb
invoke assets
invoke js
create app/assets/javascripts/blorgh/posts.js
invoke css
create app/assets/stylesheets/blorgh/posts.css
invoke css
create app/assets/stylesheets/scaffold.css
The first thing that the scaffold generator does is invoke the active_record generator, which generates a migration and a model for the resource. Note here, however, that the migration is called create_blorgh_posts rather than the usual create_posts. This is due to the isolate_namespace method called in the Blorgh::Engine class’s definition. The model here is also namespaced, being placed at app/models/blorgh/post.rb rather than app/models/post.rb.
Next, the test_unit generator is invoked for this model, generating a unit test at test/unit/blorgh/post_test.rb (rather than test/unit/post_test.rb) and a fixture at test/fixtures/blorgh/posts.yml (rather than test/fixtures/posts.yml).
After that, a line for the resource is inserted into the config/routes.rb file for the engine. This line is simply resources :posts, turning the config/routes.rb file into this:
Blorgh::Engine.routes.draw do
resources :posts
end
Note here that the routes are drawn upon the Blorgh::Engine object rather than the YourApp::Application class. This is so that the engine routes are confined to the engine itself and can be mounted at a specific point as shown in the test directory section.
Next, the scaffold_controller generator is invoked, generating a controlled called Blorgh::PostsController (at app/controllers/blorgh/posts_controller.rb) and its related views at app/views/blorgh/posts. This generator also generates a functional test for the controller (test/functional/blorgh/posts_controller_test.rb) and a helper (app/helpers/blorgh/posts_controller.rb).
Everything this generator has generated is neatly namespaced. The controller’s class is defined within the Blorgh module:
module Blorgh
class PostsController < ApplicationController
…
end
end
NOTE: The ApplicationController class being inherited from here is the Blorgh::ApplicationController, not an application’s ApplicationController.
The helper is also namespaced:
module Blorgh
class PostsHelper
…
end
end
This helps prevent conflicts with any other engine or application that may have a post resource also.
Finally, two files that are the assets for this resource are generated, app/assets/javascripts/blorgh/posts.js and app/assets/javascripts/blorgh/posts.css. You’ll see how to use these a little later.
By default, the scaffold styling is not applied to the engine as the engine’s layout file, app/views/blorgh/application.html.erb doesn’t load it. To make this apply, insert this line into the tag of this layout:
<%= stylesheet_link_tag “scaffold” %>
You can see what the engine has so far by running rake db:migrate at the root of our engine to run the migration generated by the scaffold generator, and then running rails server. When you open http://localhost:3000/blorgh/posts you will see the default scaffold that has been generated.
Click around! You’ve just generated your first engine’s first functions.
If you’d rather play around in the console, rails console will also work just like a Rails application. Remember: the Post model is namespaced, so to reference it you must call it as Blorgh::Post.
>> Blorgh::Post.find(1) => #One final thing is that the posts resource for this engine should be the root of the engine. Whenever someone goes to the root path where the engine is mounted, they should be shown a list of posts. This can be made to happen if this line is inserted into the config/routes.rb file inside the engine:
root :to => “posts#index”
Now people will only need to go to the root of the engine to see all the posts, rather than visiting /posts.
Now that the engine has the ability to create new blog posts, it only makes sense to add commenting functionality as well. To do get this, you’ll need to generate a comment model, a comment controller and then modify the posts scaffold to display comments and allow people to create new ones.
Run the model generator and tell it to generate a Comment model, with the related table having two columns: a post_id integer and text text column.
$ rails generate model Comment post_id:integer text:text
This will output the following:
invoke active_record
create db/migrate/[timestamp]create_blorgh_comments.rb
create app/models/blorgh/comment.rb
invoke test_unit
create test/unit/blorgh/commenttest.rb
create test/fixtures/blorgh/comments.yml
This generator call will generate just the necessary model files it needs, namespacing the files under a blorgh directory and creating a model class called Blorgh::Comment.
To show the comments on a post, edit app/views/posts/show.html.erb and add this line before the “Edit” link:
<%= render @post.comments %>
This line will require there to be a has_many association for comments defined on the Blorgh::Post model, which there isn’t right now. To define one, open app/models/blorgh/post.rb and add this line into the model:
has_many :comments
Turning the model into this:
module Blorgh
class Post < ActiveRecord::Base
has_many :comments
end
end
Because the has_many is defined inside a class that is inside the Blorgh module, Rails will know that you want to use the Blorgh::Comment model for these objects.
Next, there needs to be a form so that comments can be created on a post. To add this, put this line underneath the call to render @post.comments in app/views/blorgh/posts/show.html.erb:
<%= render “blorgh/comments/form” %>
Next, the partial that this line will render needs to exist. Create a new directory at app/views/blorgh/comments and in it a new file called _form.html.erb which has this content to create the required partial:
<%= form_for [@post, @post.comments.build] do |f| >
<= f.label :text >
<= f.text_area :text %>
<% end %>
This form, when submitted, is going to attempt to post to a route of posts/:post_id/comments within the engine. This route doesn’t exist at the moment, but can be created by changing the resources :posts line inside config/routes.rb into these lines:
resources :posts do
resources :comments
end
The route now will exist, but the controller that this route goes to does not. To create it, run this command:
$ rails g controller comments
This will generate the following things:
create app/controllers/blorgh/comments_controller.rb
invoke erb
exist app/views/blorgh/comments
invoke test_unit
create test/functional/blorgh/comments_controller_test.rb
invoke helper
create app/helpers/blorgh/comments_helper.rb
invoke test_unit
create test/unit/helpers/blorgh/comments_helper_test.rb
invoke assets
invoke js
create app/assets/javascripts/blorgh/comments.js
invoke css
create app/assets/stylesheets/blorgh/comments.css
The form will be making a POST request to /posts/:post_id/comments, which will correspond with the create action in Blorgh::CommentsController. This action needs to be created and can be done by putting the following lines inside the class definition in app/controllers/blorgh/comments_controller.rb:
def create
@post = Post.find(params[:post_id])
@comment = @post.comments.build(params[:comment])
flash[:notice] = “Comment has been created!”
redirect_to post_path
end
This is the final part required to get the new comment form working. Displaying the comments however, is not quite right yet. If you were to create a comment right now you would see this error:
Missing partial blorgh/comments/comment with {:handlers=>[:erb, :builder], :formats=>[:html], :locale=>[:en, :en]}. Searched in:- “/Users/ryan/Sites/side_projects/blorgh/test/dummy/app/views”
- “/Users/ryan/Sites/side_projects/blorgh/app/views”
The engine is unable to find the partial required for rendering the comments. Rails has looked firstly in the application’s (test/dummy) app/views directory and then in the engine’s app/views directory. When it can’t find it, it will throw this error. The engine knows to look for blorgh/comments/comment because the model object it is receiving is from the Blorgh::Comment class.
This partial will be responsible for rendering just the comment text, for now. Create a new file at app/views/blorgh/comments/_comment.html.erb and put this line inside it:
<%= comment_counter + 1 >. <= comment.text %>
The comment_counter local variable is given to us by the <%= render @post.comments %> call, as it will define this automatically and increment the counter as it iterates through each comment. It’s used in this example to display a small number next to each comment when it’s created.
That completes the comment function of the blogging engine. Now it’s time to use it within an application.
Using an engine within an application is very easy. First, the engine needs to be specified inside the application’s Gemfile. If there isn’t an application handy to test this out in, generate one using the rails new command outside of the engine directory like this:
$ rails new unicorn
Usually, specifying the engine inside the Gemfile would be done by specifying it as a normal, everyday gem.
gem ‘devise’
Because the blorgh engine is still under development, it will need to have a :path option for its Gemfile specification:
gem ‘blorgh’, :path => “/path/to/blorgh”
If the whole blorgh engine directory is copied to vendor/engines/blorgh then it could be specified in the Gemfile like this:
gem ‘blorgh’, :path => “vendor/engines/blorgh”
As described earlier, by placing the gem in the Gemfile it will be loaded when Rails is loaded, as it will first require lib/blorgh.rb in the engine and then lib/blorgh/engine.rb, which is the file that defines the major pieces of functionality for the engine.
To make the engine’s functionality accessible from within an application, it needs to be mounted in that application’s config/routes.rb file:
mount Blorgh::Engine, :at => “blog”NOTE: Other engines, such as Devise, handle this a little differently by making you specify custom helpers such as devise_for in the routes. These helpers do exactly the same thing, mounting pieces of the engines’s functionality at a pre-defined path which may be customizable.
This line will mount the engine
TODO: Application will provide a User foundation class which the engine hooks into through a configuration setting, configurable in the application’s initializers. The engine will be mounted at the /blog path in the application.
TODO: Cover how to override engine functionality in the engine, such as controllers and views.
IDEA: I like Devise’s devise :controllers => { “sessions” => “sessions” } idea. Perhaps we could incorporate that into the guide?
TODO: Mention how to use assets within an engine?
TODO: Mention how to depend on external gems, like RedCarpet.