Getting Started with Engines

1 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 behavior from Rails::Engine.

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

Engines are also closely related to plugins. The two share a common lib directory structure, and are both generated using the rails plugin new generator. The difference is that an engine is considered a "full plugin" by Rails (as indicated by the --full option that's passed to the generator command). We'll actually be using the --mountable option here, which includes all the features of --full, and then some. This guide will refer to these "full plugins" 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". This engine will provide blogging functionality to its host applications, allowing for new articles 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 articles_path and use an engine that also provides a path also called articles_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 its environment. 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 Thredded, an engine that provides forum functionality. There's also Spree which provides an e-commerce platform, and Refinery CMS, 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!

2 Generating an Engine

To generate an engine, 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 --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. The --full option tells the generator that you want to create an engine, including a skeleton structure that provides the following:

• An app directory tree

• A config/routes.rb file:

  Rails.application.routes.draw do
  end
  

  Copy

• 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
  

  Copy

The --mountable option will add to the --full option:

• Asset manifest files (blorgh_manifest.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
  

  Copy

• Namespace isolation to lib/blorgh/engine.rb:

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

  Copy

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 => "/blorgh"

2.1 Inside an Engine

2.1.1 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: "engines/blorgh"

Don't forget to run bundle install as usual. 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 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 is helpers. Without calling isolate_namespace, the engine's helpers would be included in an application's controllers.

What this isolation of the namespace means is that a model generated by a call to bin/rails generate model, such as bin/rails generate model article, won't be called Article, but instead be namespaced and called Blorgh::Article. In addition, the table for the model is namespaced, becoming blorgh_articles, rather than simply articles. Similar to the model namespacing, a controller called ArticlesController becomes Blorgh::ArticlesController and the views for that controller will not be at app/views/articles, but app/views/blorgh/articles instead. Mailers, jobs and helpers 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.

2.1.2 app Directory

Inside the app directory are the standard assets, controllers, helpers, jobs, mailers, models, and views directories that you should be familiar with from an application. 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 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 that contains 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.

Just like for app/controllers, you will find a blorgh subdirectory under the app/helpers, app/jobs, app/mailers and app/models directories containing the associated application_*.rb file for gathering common functionalities. By placing your files under this subdirectory and namespacing your objects, you prevent them from possibly clashing with identically-named elements within other engines or even within the application.

Lastly, the app/views directory contains a layouts folder, which contains a file at blorgh/application.html.erb. This file 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.

2.1.3 bin Directory

This directory contains one file, bin/rails, which enables you to use the rails sub-commands and generators just like you would within an application. This means that you will be able to generate new controllers and models for this engine very easily by running commands like this:

$ bin/rails generate model

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

2.1.4 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.

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

3 Providing Engine Functionality

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

3.1 Generating an Article Resource

The first thing to generate for a blog engine is the Article model and related controller. To quickly generate this, you can use the Rails scaffold generator.

$ bin/rails generate scaffold article title:string text:text

This command will output this information:

invoke  active_record
create    db/migrate/[timestamp]_create_blorgh_articles.rb
create    app/models/blorgh/article.rb
invoke    test_unit
create      test/models/blorgh/article_test.rb
create      test/fixtures/blorgh/articles.yml
invoke  resource_route
 route    resources :articles
invoke  scaffold_controller
create    app/controllers/blorgh/articles_controller.rb
invoke    erb
create      app/views/blorgh/articles
create      app/views/blorgh/articles/index.html.erb
create      app/views/blorgh/articles/edit.html.erb
create      app/views/blorgh/articles/show.html.erb
create      app/views/blorgh/articles/new.html.erb
create      app/views/blorgh/articles/_form.html.erb
create      app/views/blorgh/articles/_article.html.erb
invoke    resource_route
invoke    test_unit
create      test/controllers/blorgh/articles_controller_test.rb
create      test/system/blorgh/articles_test.rb
invoke    helper
create      app/helpers/blorgh/articles_helper.rb
invoke      test_unit

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_articles rather than the usual create_articles. 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/article.rb rather than app/models/article.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/article_test.rb (rather than test/models/article_test.rb) and a fixture at test/fixtures/blorgh/articles.yml (rather than test/fixtures/articles.yml).

After that, a line for the resource is inserted into the config/routes.rb file for the engine. This line is simply resources :articles, turning the config/routes.rb file for the engine into this:

Blorgh::Engine.routes.draw do
  resources :articles
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. It also causes the engine's routes to be isolated from those routes that are within the application. The Routes section of this guide describes it in detail.

Next, the scaffold_controller generator is invoked, generating a controller called Blorgh::ArticlesController (at app/controllers/blorgh/articles_controller.rb) and its related views at app/views/blorgh/articles. This generator also generates tests for the controller (test/controllers/blorgh/articles_controller_test.rb and test/system/blorgh/articles_test.rb) and a helper (app/helpers/blorgh/articles_helper.rb).

Everything this generator has created is neatly namespaced. The controller's class is defined within the Blorgh module:

module Blorgh
  class ArticlesController < ApplicationController
    # ...
  end
end

The helper inside app/helpers/blorgh/articles_helper.rb is also namespaced:

module Blorgh
  module ArticlesHelper
    # ...
  end
end

This helps prevent conflicts with any other engine or application that may have an article resource as well.

You can see what the engine has so far by running bin/rails db:migrate at the root of our engine to run the migration generated by the scaffold generator, and then running bin/rails server in test/dummy. When you open http://localhost:3000/blorgh/articles 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, bin/rails console will also work just like a Rails application. Remember: the Article model is namespaced, so to reference it you must call it as Blorgh::Article.

irb> Blorgh::Article.find(1)
=> #<Blorgh::Article id: 1 ...>

One final thing is that the articles 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 articles. This can be made to happen if this line is inserted into the config/routes.rb file inside the engine:

root to: "articles#index"

Now people will only need to go to the root of the engine to see all the articles, rather than visiting /articles. This means that instead of http://localhost:3000/blorgh/articles, you only need to go to http://localhost:3000/blorgh now.

3.2 Generating a Comments Resource

Now that the engine can create new articles, it only makes sense to add commenting functionality as well. To do this, you'll need to generate a comment model, a comment controller, and then modify the articles scaffold to display comments and allow people to create new ones.

From the engine root, run the model generator. Tell it to generate a Comment model, with the related table having two columns: an article_id integer and text text column.

$ bin/rails generate model Comment article_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. Now run the migration to create our blorgh_comments table:

$ bin/rails db:migrate

To show the comments on an article, edit app/views/blorgh/articles/show.html.erb and add this line before the "Edit" link:

<h3>Comments</h3>
<%= render @article.comments %>

This line will require there to be a has_many association for comments defined on the Blorgh::Article model, which there isn't right now. To define one, open app/models/blorgh/article.rb and add this line into the model:

has_many :comments

Turning the model into this:

module Blorgh
  class Article < ApplicationRecord
    has_many :comments
  end
end

Next, there needs to be a form so that comments can be created on an article. To add this, put this line underneath the call to render @article.comments in app/views/blorgh/articles/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_with model: [@article, @article.comments.build] do |form| %>
  <p>
    <%= form.label :text %><br>
    <%= form.text_area :text %>
  </p>
  <%= form.submit %>
<% end %>

When this form is submitted, it is going to attempt to perform a POST request to a route of /articles/:article_id/comments within the engine. This route doesn't exist at the moment, but can be created by changing the resources :articles line inside config/routes.rb into these lines:

resources :articles 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 from the engine root:

$ bin/rails generate 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

The form will be making a POST request to /articles/:article_id/comments, which will correspond with the create action in Blorgh::CommentsController. This action needs to be created, which can be done by putting the following lines inside the class definition in app/controllers/blorgh/comments_controller.rb:

def create
  @article = Article.find(params[:article_id])
  @comment = @article.comments.create(comment_params)
  flash[:notice] = "Comment has been created!"
  redirect_to articles_path
end

private
  def comment_params
    params.require(:comment).permit(:text)
  end

This is the final step 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 @article.comments %> call, which will define it 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.

4 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 articles and comments within the engine.

4.1 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: "engines/blorgh"

Then run bundle to install the gem.

As described earlier, by placing the gem in the Gemfile it will be loaded when Rails is loaded. It will first require lib/blorgh.rb from the engine, 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 bin/rails server.

4.2 Engine Setup

The engine contains migrations for the blorgh_articles 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 run the following command from the application's root:

$ bin/rails blorgh:install:migrations

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

$ bin/rails railties:install:migrations

You can specify a custom path in the source engine for the migrations by specifying MIGRATIONS_PATH.

$ bin/rails railties:install:migrations MIGRATIONS_PATH=db_blourgh

If you have multiple databases you can also specify the target database by specifying DATABASE.

$ bin/rails railties:install:migrations DATABASE=animals

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_articles.blorgh.rb from blorgh
Copied migration [timestamp_2]_create_blorgh_comments.blorgh.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 bin/rails db:migrate. When accessing the engine through http://localhost:3000/blog, the articles 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:

$ bin/rails db:migrate SCOPE=blorgh

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

$ bin/rails db:migrate SCOPE=blorgh VERSION=0

4.3 Using a Class Provided by the Application

4.3.1 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 articles 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 an article 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 that represents the users of the application (we'll get into making this configurable further on). It can be generated using this command inside the application:

$ bin/rails generate model user name:string

The bin/rails 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 articles 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 either create a new User object from it, or find one that already has that name. The engine will then associate the article with the found or created User object.

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

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

Next, we need to update our Blorgh::ArticlesController#article_params method to permit the new form parameter:

def article_params
  params.require(:article).permit(:title, :text, :author_name)
end

The Blorgh::Article model should then have some code to convert the author_name field into an actual User object and associate it as that article's author before the article is saved. It will also need to have an attr_accessor set up 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_validation call into app/models/blorgh/article.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_validation :set_author

private
  def set_author
    self.author = User.find_or_create_by(name: author_name)
  end

By representing the author association's object with 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_articles 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_articles table.

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

$ bin/rails generate migration add_author_id_to_blorgh_articles author_id:integer

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

$ bin/rails blorgh:install:migrations

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

Run the migration using:

$ bin/rails 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 an article, represented by the blorgh_articles table from the engine.

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

<p>
  <strong>Author:</strong>
  <%= article.author.name %>
</p>

4.3.2 Using a Controller Provided by the Application

Because Rails controllers generally share code for things like authentication and accessing session variables, they inherit from ApplicationController by default. 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 need to 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:

module Blorgh
  class ApplicationController < ::ApplicationController
  end
end

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

4.4 Configuring an Engine

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

4.4.1 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 that class may not always be User, as previously explained. To make this setting customizable, the engine will have a configuration setting called author_class that will be used to specify which class represents users inside the application.

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

mattr_accessor :author_class

This method works like its siblings, 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.author_class.

The next step is to switch the Blorgh::Article model over to this new setting. Change the belongs_to association inside this model (app/models/blorgh/article.rb) to this:

belongs_to :author, class_name: Blorgh.author_class

The set_author method in the Blorgh::Article model should also use this class:

self.author = Blorgh.author_class.constantize.find_or_create_by(name: author_name)

To save having to call constantize on the author_class result all the time, you could instead just override the author_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.author_class
  @@author_class.constantize
end

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

self.author = Blorgh.author_class.find_or_create_by(name: author_name)

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

Since we changed the author_class method to return a Class instead of a String, we must also modify our belongs_to definition in the Blorgh::Article model:

belongs_to :author, class_name: Blorgh.author_class.to_s

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.author_class = "User"

Go ahead and try to create a new article. 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 method which returns an object of that class, to be associated with an article when it's created. This object, of course, should have some sort of identifier by which it can be referenced.

4.4.2 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. The same thing goes 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.

5 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.

5.1 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:

module Blorgh
  class FooControllerTest < ActionDispatch::IntegrationTest
    include Engine.routes.url_helpers

    def test_index
      get foos_url
      # ...
    end
  end
end

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 set the @routes instance variable to the engine's route set in your setup code:

module Blorgh
  class FooControllerTest < ActionDispatch::IntegrationTest
    include Engine.routes.url_helpers

    setup do
      @routes = Engine.routes
    end

    def test_index
      get foos_url
      # ...
    end
  end
end

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

This also ensures that the engine's URL helpers will work as expected in your tests.

6 Improving Engine Functionality

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

6.1 Overriding Models and Controllers

Engine models and controllers can be reopened by the parent application to extend or decorate them.

Overrides may be organized in a dedicated directory app/overrides, ignored by the autoloader, and preloaded in a to_prepare callback:

# config/application.rb
module MyApp
  class Application < Rails::Application
    # ...

    overrides = "#{Rails.root}/app/overrides"
    Rails.autoloaders.main.ignore(overrides)

    config.to_prepare do
      Dir.glob("#{overrides}/**/*_override.rb").sort.each do |override|
        load override
      end
    end
  end
end

6.1.1 Reopening Existing Classes Using class_eval

For example, in order to override the engine model

# Blorgh/app/models/blorgh/article.rb
module Blorgh
  class Article < ApplicationRecord
    # ...
  end
end

you just create a file that reopens that class:

# MyApp/app/overrides/models/blorgh/article_override.rb
Blorgh::Article.class_eval do
  # ...
end

It is very important that the override reopens the class or module. Using the class or module keywords would define them if they were not already in memory, which would be incorrect because the definition lives in the engine. Using class_eval as shown above ensures you are reopening.

6.1.2 Reopening Existing Classes 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. ActiveSupport::Concern manages load order of interlinked dependent modules and classes at run time allowing you to significantly modularize your code.

Adding Article#time_since_created and Overriding Article#summary:

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

class Blorgh::Article < ApplicationRecord
  include Blorgh::Concerns::Models::Article

  def time_since_created
    Time.current - created_at
  end

  def summary
    "#{title} - #{truncate(text)}"
  end
end

# Blorgh/app/models/blorgh/article.rb
module Blorgh
  class Article < ApplicationRecord
    include Blorgh::Concerns::Models::Article
  end
end

# Blorgh/lib/concerns/models/article.rb

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

  # `included do` causes the block to be evaluated in the context
  # in which the module is included (i.e. Blorgh::Article),
  # rather than in the module itself.
  included do
    attr_accessor :author_name
    belongs_to :author, class_name: "User"

    before_validation :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

6.2 Autoloading and Engines

Please check the Autoloading and Reloading Constants guide for more information about autoloading and engines.

6.3 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, it will check in the app/views directories of all engines that have this directory.

When the application is asked to render the view for Blorgh::ArticlesController's index action, it will first look for the path app/views/blorgh/articles/index.html.erb within the application. If it cannot find it, it will look inside the engine.

You can override this view in the application by simply creating a new file at app/views/blorgh/articles/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/articles/index.html.erb and put this content in it:

<h1>Articles</h1>
<%= link_to "New Article", new_article_path %>
<% @articles.each do |article| %>
  <h2><%= article.title %></h2>
  <small>By <%= article.author %></small>
  <%= simple_format(article.text) %>
  <hr>
<% end %>

6.4 Routes

Routes inside an engine are isolated from the application by default. 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 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 :articles
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 articles_path may end up going to undesired locations if both the application and the engine have such a helper defined.

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

<%= link_to "Blog articles", articles_path %>

To make this route always use the engine's articles_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 articles", blorgh.articles_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 rendered from within an engine attempts 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.

6.5 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 assets in the engine's app/assets and lib/assets directories.

Like all of the other components of an engine, the assets should be namespaced. This means that 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 isn't namespaced, 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 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 Asset Pipeline require statements in processed files:

/*
 *= require blorgh/style
 */

6.6 Separate Assets and 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 "blorgh/admin.css" in its stylesheets. In this situation, you should explicitly define these assets for precompilation. This tells Sprockets to add your engine assets when bin/rails assets:precompile is triggered.

You can define assets for precompilation in engine.rb:

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

For more information, read the Asset Pipeline guide.

6.7 Other Gem Dependencies

Gem dependencies inside an engine should be specified inside the .gemspec file at the root of the engine. The reason is that the engine may be installed as a gem. If dependencies were to be specified inside the Gemfile, these would not be recognized 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 of the application. The development dependencies for the gem will only be used when the development and 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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