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Getting Started with Engines

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

What are engines?

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 a lot of its behaviour from Rails::Engine.

Therefore, engines and applications can be thought of almost the same thing, just with very minor differences, as you'll see throughout this guide. Engines and applications also share a common structure.

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 difference being that an engine is considered a "full plugin" by Rails as indicated by the --full option that's passed to the generator command, but this guide will refer to them simply as "engines" throughout. An engine can be a plugin, and a plugin can be an engine.

The engine that will be created in 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. At the beginning of this guide, you will be working solely within the engine itself, but 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.

It's important to keep in mind at all times that the application should always take precedence over its engines. An application is the object that has final say in what goes on in the universe (with the universe being the application's environment) where the engine should only be enhancing it, rather than changing it drastically.

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. There's also Spree which provides an e-commerce platform, and RefineryCMS, a CMS engine.

Finally, engines would not have been 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!

Generating an engine

To generate an engine with Rails 3.2, you will need to run the plugin generator and pass it options as appropriate to the need. For the "blorgh" example, you will need to create a "mountable" engine, running this command in a terminal:

$ rails plugin new blorgh --mountable

The full list of options for the plugin generator may be seen by typing:

$ rails plugin --help

The --full option tells the generator that you want to create an engine, including a skeleton structure by providing the following:

  • An app directory tree
  • A config/routes.rb file:

    Rails.application.routes.draw do
    end
    
  • A file at lib/blorgh/engine.rb which is identical in function to a standard Rails application's config/application.rb file:

    module Blorgh
      class Engine < ::Rails::Engine
      end
    end
    

The --mountable option tells the generator that you want to create a "mountable" and namespace-isolated engine. This generator will provide the same skeleton structure as would the --full option, and will add:

  • Asset manifest files (application.js and application.css)
  • A namespaced ApplicationController stub
  • A namespaced ApplicationHelper stub
  • A layout view template for the engine
  • Namespace isolation to config/routes.rb:

    Blorgh::Engine.routes.draw do
    end
    
  • Namespace isolation to lib/blorgh/engine.rb:

    module Blorgh
      class Engine < ::Rails::Engine
        isolate_namespace Blorgh
      end
    end
    

Additionally, the --mountable option tells the generator to mount the engine inside the dummy testing application located at test/dummy by adding the following to the dummy application's routes file at test/dummy/config/routes.rb:

mount Blorgh::Engine, :at => "blorgh"

Inside an engine

Critical files

At the root of this brand new engine's directory lives a blorgh.gemspec file. When you include the engine into an application later on, you will do so with this line in the 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

TIP: Some engines choose to use this file to put global configuration options for their engine. It's a relatively good idea, and so if you want to offer configuration options, the file where your engine's module is defined is perfect for that. Place the methods inside the module and you'll be good to go.

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 gem notifies Rails that there's an engine at the specified path, and will correctly mount the engine inside the application, performing tasks such as adding the app directory of the engine to the load path for models, mailers, controllers and views.

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, away from similar components inside the application. Without this, there is a possibility that the engine's components could "leak" into the application, causing unwanted disruption, or that important engine components could be overridden by similarly named things within the application. One of the examples of such conflicts are helpers. Without calling isolate_namespace, engine's helpers would be included in an application's controllers.

NOTE: It is highly recommended that the isolate_namespace line be left within the Engine class definition. Without it, classes generated in an engine may conflict with an application.

What this isolation of the namespace means is that a model generated by a call to rails g model such as rails g model post won't be called Post, but instead be namespaced and called Blorgh::Post. In addition, the table for the model is namespaced, becoming blorgh_posts, rather than simply posts. Similar to the model namespacing, a controller called PostsController becomes Blorgh::PostsController and the views for that controller will not be at app/views/posts, but app/views/blorgh/posts instead. Mailers are namespaced as well.

Finally, routes will also be isolated within the engine. This is one of the most important parts about namespacing, and is discussed later in the Routes section of this guide.

app directory

Inside the app directory are 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, when we're writing the engine.

Within the app/assets directory, there are the images, javascripts and stylesheets directories which, again, you should be familiar with due to their similarity to 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.

NOTE: The ApplicationController class inside an engine is named just like a Rails application in order to make it easier for you to convert your applications into engines.

Lastly, the app/views directory contains a layouts folder which contains a 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 application's app/views/layouts/application.html.erb file.

If you don't want to force a layout on to users of the engine, then you can delete this file and reference a different layout in the controllers of your engine.

script directory

This directory contains one file, script/rails, which enables 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 by running commands like this:

rails g model

Keeping in mind, of course, that anything generated with these commands inside an engine that has isolate_namespace inside the Engine class will be namespaced.

test directory

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 /blorgh, which will make it accessible through the application only at that path.

Also in the test directory is the test/integration directory, where integration tests for the engine should be placed. Other directories can be created in the test directory also. For example, you may wish to create a test/models directory for your models tests.

Providing engine functionality

The engine that this guide covers provides posting and commenting functionality and follows a similar thread to the Getting Started Guide, with some new twists.

Generating a post resource

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/models/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/controllers/blorgh/posts_controller_test.rb
invoke    helper
create      app/helpers/blorgh/posts_helper.rb
invoke      test_unit
create        test/helpers/blorgh/posts_helper_test.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 due to the isolate_namespace call within the Engine class.

Next, the test_unit generator is invoked for this model, generating a model test at test/models/blorgh/post_test.rb (rather than test/models/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 for the engine 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. This is also what causes the engine's routes to be isolated from those routes that are within the application. This is discussed further in the Routes section of this guide.

Next, the scaffold_controller generator is invoked, generating a controller called Blorgh::PostsController (at app/controllers/blorgh/posts_controller.rb) and its related views at app/views/blorgh/posts. This generator also generates a test for the controller (test/controllers/blorgh/posts_controller_test.rb) and a helper (app/helpers/blorgh/posts_controller.rb).

Everything this generator has created 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 inside app/helpers/blorgh/posts_helper.rb 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 <head> 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 in test/dummy. 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)
=> #<Blorgh::Post id: 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. This means that instead of http://localhost:3000/blorgh/posts, you only need to go to http://localhost:3000/blorgh now.

Generating a comments resource

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/models/blorgh/comment_test.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/blorgh/posts/show.html.erb and add this line before the "Edit" link:

<h3>Comments</h3>
<%= 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

NOTE: 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, so there's no need to specify that using the :class_name option here.

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:

<h3>New comment</h3>
<%= form_for [@post, @post.comments.build] do |f| %>
  <p>
    <%= f.label :text %><br />
    <%= f.text_area :text %>
  </p>
  <%= f.submit %>
<% end %>

When this form is submitted, it is going to attempt to perform a POST request 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

This creates a nested route for the comments, which is what the form requires.

The route now exists, 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/controllers/blorgh/comments_controller_test.rb
invoke  helper
create    app/helpers/blorgh/comments_helper.rb
invoke    test_unit
create      test/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.create(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 looks first 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.

Hooking into an application

Using an engine within an application is very easy. This section covers how to mount the engine into an application and the initial setup required, as well as linking the engine to a User class provided by the application to provide ownership for posts and comments within the engine.

Mounting the engine

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'

However, because you are developing the blorgh engine on your local machine, you will need to specify the :path option in your Gemfile:

gem 'blorgh', :path => "/path/to/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"

This line will mount the engine at /blog in the application. Making it accessible at http://localhost:3000/blog when the application runs with rails server.

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.

Engine setup

The engine contains migrations for the blorgh_posts and blorgh_comments table which need to be created in the application's database so that the engine's models can query them correctly. To copy these migrations into the application use this command:

$ rake blorgh:install:migrations

If you have multiple engines that need migrations copied over, use railties:install:migrations instead:

$ rake railties:install:migrations

This command, when run for the first time will copy over all the migrations from the engine. When run the next time, it will only copy over migrations that haven't been copied over already. The first run for this command will output something such as this:

Copied migration [timestamp_1]_create_blorgh_posts.rb from blorgh
Copied migration [timestamp_2]_create_blorgh_comments.rb from blorgh

The first timestamp ([timestamp_1]) will be the current time and the second timestamp ([timestamp_2]) will be the current time plus a second. The reason for this is so that the migrations for the engine are run after any existing migrations in the application.

To run these migrations within the context of the application, simply run rake db:migrate. When accessing the engine through http://localhost:3000/blog, the posts will be empty. This is because the table created inside the application is different from the one created within the engine. Go ahead, play around with the newly mounted engine. You'll find that it's the same as when it was only an engine.

If you would like to run migrations only from one engine, you can do it by specifying SCOPE:

rake db:migrate SCOPE=blorgh

This may be useful if you want to revert engine's migrations before removing it. In order to revert all migrations from blorgh engine you can run such code:

rake db:migrate SCOPE=blorgh VERSION=0

Using a class provided by the application

Using a model provided by the application

When an engine is created, it may want to use specific classes from an application to provide links between the pieces of the engine and the pieces of the application. In the case of the blorgh engine, making posts and comments have authors would make a lot of sense.

A typical application might have a User class that would be used to represent authors for a post or a comment. But there could be a case where the application calls this class something different, such as Person. For this reason, the engine should not hardcode associations specifically for a User class.

To keep it simple in this case, the application will have a class called User which will represent the users of the application. It can be generated using this command inside the application:

rails g model user name:string

The rake db:migrate command needs to be run here to ensure that our application has the users table for future use.

Also, to keep it simple, the posts form will have a new text field called author_name where users can elect to put their name. The engine will then take this name and create a new User object from it or find one that already has that name, and then associate the post with it.

First, the author_name text field needs to be added to the app/views/blorgh/posts/_form.html.erb partial inside the engine. This can be added above the title field with this code:

<div class="field">
  <%= f.label :author_name %><br />
  <%= f.text_field :author_name %>
</div>

The Blorgh::Post model should then have some code to convert the author_name field into an actual User object and associate it as that post's author before the post is saved. It will also need to have an attr_accessor setup for this field so that the setter and getter methods are defined for it.

To do all this, you'll need to add the attr_accessor for author_name, the association for the author and the before_save call into app/models/blorgh/post.rb. The author association will be hard-coded to the User class for the time being.

attr_accessor :author_name
belongs_to :author, :class_name => "User"

before_save :set_author

private
  def set_author
    self.author = User.find_or_create_by_name(author_name)
  end

By defining that the author association's object is represented by the User class a link is established between the engine and the application. There needs to be a way of associating the records in the blorgh_posts table with the records in the users table. Because the association is called author, there should be an author_id column added to the blorgh_posts table.

To generate this new column, run this command within the engine:

$ rails g migration add_author_id_to_blorgh_posts author_id:integer

NOTE: Due to the migration's name and the column specification after it, Rails will automatically know that you want to add a column to a specific table and write that into the migration for you. You don't need to tell it any more than this.

This migration will need to be run on the application. To do that, it must first be copied using this command:

$ rake blorgh:install:migrations

Notice here that only one migration was copied over here. This is because the first two migrations were copied over the first time this command was run.

NOTE Migration [timestamp]_create_blorgh_posts.rb from blorgh has been skipped. Migration with the same name already exists.
NOTE Migration [timestamp]_create_blorgh_comments.rb from blorgh has been skipped. Migration with the same name already exists.
Copied migration [timestamp]_add_author_id_to_blorgh_posts.rb from blorgh

Run this migration using this command:

$ rake db:migrate

Now with all the pieces in place, an action will take place that will associate an author -- represented by a record in the users table -- with a post, represented by the blorgh_posts table from the engine.

Finally, the author's name should be displayed on the post's page. Add this code above the "Title" output inside app/views/blorgh/posts/show.html.erb:

<p>
  <b>Author:</b>
  <%= @post.author %>
</p>

By outputting @post.author using the <%= tag, the to_s method will be called on the object. By default, this will look quite ugly:

#<User:0x00000100ccb3b0>

This is undesirable and it would be much better to have the user's name there. To do this, add a to_s method to the User class within the application:

def to_s
  name
end

Now instead of the ugly Ruby object output the author's name will be displayed.

Using a controller provided by the application

Because Rails controllers generally share code for things like authentication and accessing session variables, by default they inherit from ApplicationController. Rails engines, however are scoped to run independently from the main application, so each engine gets a scoped ApplicationController. This namespace prevents code collisions, but often engine controllers should access methods in the main application's ApplicationController. An easy way to provide this access is to change the engine's scoped ApplicationController to inherit from the main application's ApplicationController. For our Blorgh engine this would be done by changing app/controllers/blorgh/application_controller.rb to look like:

class Blorgh::ApplicationController < ApplicationController
end

By default, the engine's controllers inherit from Blorgh::ApplicationController. So, after making this change they will have access to the main applications ApplicationController as though they were part of the main application.

This change does require that the engine is run from a Rails application that has an ApplicationController.

Configuring an engine

This section covers how to make the User class configurable, followed by general configuration tips for the engine.

Setting configuration settings in the application

The next step is to make the class that represents a User in the application customizable for the engine. This is because, as explained before, that class may not always be User. To make this customizable, the engine will have a configuration setting called user_class that will be used to specify what the class representing users is inside the application.

To define this configuration setting, you should use a mattr_accessor inside the Blorgh module for the engine, located at lib/blorgh.rb inside the engine. Inside this module, put this line:

mattr_accessor :user_class

This method works like its brothers attr_accessor and cattr_accessor, but provides a setter and getter method on the module with the specified name. To use it, it must be referenced using Blorgh.user_class.

The next step is switching the Blorgh::Post model over to this new setting. For the belongs_to association inside this model (app/models/blorgh/post.rb), it will now become this:

belongs_to :author, :class_name => Blorgh.user_class

The set_author method also located in this class should also use this class:

self.author = Blorgh.user_class.constantize.find_or_create_by_name(author_name)

To save having to call constantize on the user_class result all the time, you could instead just override the user_class getter method inside the Blorgh module in the lib/blorgh.rb file to always call constantize on the saved value before returning the result:

def self.user_class
  @@user_class.constantize
end

This would then turn the above code for set_author into this:

self.author = Blorgh.user_class.find_or_create_by_name(author_name)

Resulting in something a little shorter, and more implicit in its behaviour. The user_class method should always return a Class object.

To set this configuration setting within the application, an initializer should be used. By using an initializer, the configuration will be set up before the application starts and calls the engine's models which may depend on this configuration setting existing.

Create a new initializer at config/initializers/blorgh.rb inside the application where the blorgh engine is installed and put this content in it:

Blorgh.user_class = "User"

WARNING: It's very important here to use the String version of the class, rather than the class itself. If you were to use the class, Rails would attempt to load that class and then reference the related table, which could lead to problems if the table wasn't already existing. Therefore, a String should be used and then converted to a class using constantize in the engine later on.

Go ahead and try to create a new post. You will see that it works exactly in the same way as before, except this time the engine is using the configuration setting in config/initializers/blorgh.rb to learn what the class is.

There are now no strict dependencies on what the class is, only what the API for the class must be. The engine simply requires this class to define a find_or_create_by_name method which returns an object of that class to be associated with a post when it's created. This object, of course, should have some sort of identifier by which it can be referenced.

General engine configuration

Within an engine, there may come a time where you wish to use things such as initializers, internationalization or other configuration options. The great news is that these things are entirely possible because a Rails engine shares much the same functionality as a Rails application. In fact, a Rails application's functionality is actually a superset of what is provided by engines!

If you wish to use an initializer -- code that should run before the engine is loaded -- the place for it is the config/initializers folder. This directory's functionality is explained in the Initializers section of the Configuring guide, and works precisely the same way as the config/initializers directory inside an application. Same goes for if you want to use a standard initializer.

For locales, simply place the locale files in the config/locales directory, just like you would in an application.

Testing an engine

When an engine is generated there is a smaller dummy application created inside it at test/dummy. This application is used as a mounting point for the engine to make testing the engine extremely simple. You may extend this application by generating controllers, models or views from within the directory, and then use those to test your engine.

The test directory should be treated like a typical Rails testing environment, allowing for unit, functional and integration tests.

Functional tests

A matter worth taking into consideration when writing functional tests is that the tests are going to be running on an application -- the test/dummy application -- rather than your engine. This is due to the setup of the testing environment; an engine needs an application as a host for testing its main functionality, especially controllers. This means that if you were to make a typical GET to a controller in a controller's functional test like this:

get :index

It may not function correctly. This is because the application doesn't know how to route these requests to the engine unless you explicitly tell it how. To do this, you must pass the :use_route option (as a parameter) on these requests also:

get :index, :use_route => :blorgh

This tells the application that you still want to perform a GET request to the index action of this controller, just that you want to use the engine's route to get there, rather than the application.

Improving engine functionality

This section explains how to add and/or override engine MVC functionality in the main Rails application.

Overriding Models and Controllers

Engine model and controller classes can be extended by open classing them in the main Rails application (since model and controller classes are just Ruby classes that inherit Rails specific functionality). Open classing an Engine class redefines it for use in the main applicaiton. This is usually implemented by using the decorator pattern.

For simple class modifications use Class#class_eval, and for complex class modifications, consider using ActiveSupport::Concern.

Implementing Decorator Pattern Using Class#class_eval

Adding Post#time_since_created,

# MyApp/app/decorators/models/blorgh/post_decorator.rb

Blorgh::Post.class_eval do
  def time_since_created
    Time.current - created_at
  end
end
# Blorgh/app/models/post.rb

class Post < ActiveRecord::Base
  has_many :comments
end

Overriding Post#summary

# MyApp/app/decorators/models/blorgh/post_decorator.rb

Blorgh::Post.class_eval do
  def summary
    "#{title} - #{truncate(text)}"
  end
end
# Blorgh/app/models/post.rb

class Post < ActiveRecord::Base
  has_many :comments
  def summary
    "#{title}"
  end
end

Implementing Decorator Pattern Using ActiveSupport::Concern

Using Class#class_eval is great for simple adjustments, but for more complex class modifications, you might want to consider using ActiveSupport::Concern helps manage load order of interlinked dependencies at run time allowing you to significantly modularize your code.

Adding Post#time_since_created
Overriding Post#summary

# MyApp/app/models/blorgh/post.rb

class Blorgh::Post < ActiveRecord::Base
  include Blorgh::Concerns::Models::Post

  def time_since_created
    Time.current - created_at
  end

  def summary
    "#{title} - #{truncate(text)}"
  end
end
# Blorgh/app/models/post.rb

class Post < ActiveRecord::Base
  include Blorgh::Concerns::Models::Post
end
# Blorgh/lib/concerns/models/post

module Blorgh::Concerns::Models::Post
  extend ActiveSupport::Concern

  # 'included do' causes the included code to be evaluated in the
  # conext where it is included (post.rb), rather than be 
  # executed in the module's context (blorgh/concerns/models/post).
  included do
    attr_accessor :author_name
    belongs_to :author, :class_name => "User"

    before_save :set_author

    private

    def set_author
      self.author = User.find_or_create_by_name(author_name)
    end
  end

  def summary
    "#{title}"
  end

  module ClassMethods
    def some_class_method
      'some class method string'
    end
  end
end

Overriding views

When Rails looks for a view to render, it will first look in the app/views directory of the application. If it cannot find the view there, then it will check in the app/views directories of all engines which have this directory.

In the blorgh engine, there is a currently a file at app/views/blorgh/posts/index.html.erb. When the engine is asked to render the view for Blorgh::PostsController's index action, it will first see if it can find it at app/views/blorgh/posts/index.html.erb within the application and then if it cannot it will look inside the engine.

You can override this view in the application by simply creating a new file at app/views/blorgh/posts/index.html.erb. Then you can completely change what this view would normally output.

Try this now by creating a new file at app/views/blorgh/posts/index.html.erb and put this content in it:

<h1>Posts</h1>
<%= link_to "New Post", new_post_path %>
<% @posts.each do |post| %>
  <h2><%= post.title %></h2>
  <small>By <%= post.author %></small>
  <%= simple_format(post.text) %>
  <hr>
<% end %>

Routes

Routes inside an engine are, by default, isolated from the application. This is done by the isolate_namespace call inside the Engine class. This essentially means that the application and its engines can have identically named routes, and that they will not clash.

Routes inside an engine are drawn on the Engine class within config/routes.rb, like this:

Blorgh::Engine.routes.draw do
  resources :posts
end

By having isolated routes such as this, if you wish to link to an area of an engine from within an application, you will need to use the engine's routing proxy method. Calls to normal routing methods such as posts_path may end up going to undesired locations if both the application and the engine both have such a helper defined.

For instance, the following example would go to the application's posts_path if that template was rendered from the application, or the engine's posts_path if it was rendered from the engine:

<%= link_to "Blog posts", posts_path %>

To make this route always use the engine's posts_path routing helper method, we must call the method on the routing proxy method that shares the same name as the engine.

<%= link_to "Blog posts", blorgh.posts_path %>

If you wish to reference the application inside the engine in a similar way, use the main_app helper:

<%= link_to "Home", main_app.root_path %>

If you were to use this inside an engine, it would always go to the application's root. If you were to leave off the main_app "routing proxy" method call, it could potentially go to the engine's or application's root, depending on where it was called from.

If a template is rendered from within an engine and it's attempting to use one of the application's routing helper methods, it may result in an undefined method call. If you encounter such an issue, ensure that you're not attempting to call the application's routing methods without the main_app prefix from within the engine.

Assets

Assets within an engine work in an identical way to a full application. Because the engine class inherits from Rails::Engine, the application will know to look up in the engine's app/assets and lib/assets directories for potential assets.

Much like all the other components of an engine, the assets should also be namespaced. This means if you have an asset called style.css, it should be placed at app/assets/stylesheets/[engine name]/style.css, rather than app/assets/stylesheets/style.css. If this asset wasn't namespaced, then there is a possibility that the host application could have an asset named identically, in which case the application's asset would take precedence and the engine's one would be all but ignored.

Imagine that you did have an asset located at app/assets/stylesheets/blorgh/style.css To include this asset inside an application, just use stylesheet_link_tag and reference the asset as if it were inside the engine:

<%= stylesheet_link_tag "blorgh/style.css" %>

You can also specify these assets as dependencies of other assets using the Asset Pipeline require statements in processed files:

/*
 *= require blorgh/style
*/

INFO. Remember that in order to use languages like Sass or CoffeeScript, you should add the relevant library to your engine's .gemspec.

Separate Assets & Precompiling

There are some situations where your engine's assets are not required by the host application. For example, say that you've created an admin functionality that only exists for your engine. In this case, the host application doesn't need to require admin.css or admin.js. Only the gem's admin layout needs these assets. It doesn't make sense for the host app to include "blorg/admin.css" in it's stylesheets. In this situation, you should explicitly define these assets for precompilation. This tells sprockets to add your engine assets when rake assets:precompile is ran.

You can define assets for precompilation in engine.rb

initializer "blorgh.assets.precompile" do |app|
  app.config.assets.precompile += %w(admin.css admin.js)
end

For more information, read the Asset Pipeline guide

Other gem dependencies

Gem dependencies inside an engine should be specified inside the .gemspec file at the root of the engine. The reason for this is because the engine may be installed as a gem. If dependencies were to be specified inside the Gemfile, these would not be recognised by a traditional gem install and so they would not be installed, causing the engine to malfunction.

To specify a dependency that should be installed with the engine during a traditional gem install, specify it inside the Gem::Specification block inside the .gemspec file in the engine:

s.add_dependency "moo"

To specify a dependency that should only be installed as a development dependency of the application, specify it like this:

s.add_development_dependency "moo"

Both kinds of dependencies will be installed when bundle install is run inside the application. The development dependencies for the gem will only be used when the tests for the engine are running.

Note that if you want to immediately require dependencies when the engine is required, you should require them before the engine's initialization. For example:

require 'other_engine/engine'
require 'yet_another_engine/engine'

module MyEngine
  class Engine < ::Rails::Engine
  end
end
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