This removes unnecessary indentation of several code snippets, and otherwise normalizes indentation of a handful more.
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DO NOT READ THIS FILE ON GITHUB, GUIDES ARE PUBLISHED ON https://guides.rubyonrails.org.
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.
After reading this guide, you will know:
- What makes an engine.
- How to generate an engine.
- How to build features for the engine.
- How to hook the engine into an application.
- How to override engine functionality in the application.
- Avoid loading Rails frameworks with Load and Configuration Hooks
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!
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
-
A file at
lib/blorgh/engine.rb
, which is identical in function to a standard Rails application'sconfig/application.rb
file:module Blorgh class Engine < ::Rails::Engine end end
The --mountable
option will add to the --full
option:
-
Asset manifest files (
blorgh_manifest.js
andapplication.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 => "/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: '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
TIP: Some engines choose to use this file to put global configuration options
for their engine. It's a relatively good idea, 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 is helpers. Without calling
isolate_namespace
, the 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 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.
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.
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.
NOTE: Because of the way that Ruby does constant lookup you may run into a situation
where your engine controller is inheriting from the main application controller and
not your engine's application controller. Ruby is able to resolve the ApplicationController
constant, and therefore the autoloading mechanism is not triggered. See the section When Constants Aren't Missed.
The best way to prevent this from happening is to use require_dependency
to ensure that the engine's application
controller is loaded. For example:
# app/controllers/blorgh/articles_controller.rb:
require_dependency "blorgh/application_controller"
module Blorgh
class ArticlesController < ApplicationController
...
end
end
WARNING: Don't use require
because it will break the automatic reloading of classes
in the development environment - using require_dependency
ensures that classes are
loaded and unloaded in the correct manner.
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.
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.
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.
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.
NOTE: For this section, make sure to run the commands in the root of the
blorgh
engine's directory.
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
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
invoke assets
invoke css
create app/assets/stylesheets/blorgh/articles.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_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
NOTE: The ArticlesController
class inherits from
Blorgh::ApplicationController
, not the application's ApplicationController
.
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.
Finally, the assets for this resource are generated in one file: app/assets/stylesheets/blorgh/articles.css
. You'll see how to use these a little later.
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.
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
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 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
invoke assets
invoke css
create app/assets/stylesheets/blorgh/comments.css
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.
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.
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
.
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_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
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
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 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
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:
$ 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/show.html.erb
:
<p>
<b>Author:</b>
<%= @article.author.name %>
</p>
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
.
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 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"
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. This could lead to
problems if the table didn't already exist. 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 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.
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.
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:
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.
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 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
that is preloaded in a to_prepare
callback.
In zeitwerk
mode you'd do this:
# 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").each do |override|
load override
end
end
end
end
and in classic
mode this:
# config/application.rb
module MyApp
class Application < Rails::Application
...
config.to_prepare do
Dir.glob("#{Rails.root}/app/overrides/**/*_override.rb").each do |override|
require_dependency override
end
end
end
end
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
has_many :comments
def summary
"#{title}"
end
end
end
you just create a file that reopens that class:
# MyApp/app/overrides/models/blorgh/article_override.rb
Blorgh::Article.class_eval do
def time_since_created
Time.current - created_at
end
def summary
"#{title} - #{truncate(text)}"
end
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.
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
]
(https://api.rubyonrails.org/classes/ActiveSupport/Concern.html).
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 included code to be evaluated in the
# context where it is included (article.rb), rather than being
# executed in the module's context (blorgh/concerns/models/article).
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
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 %>
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.
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
*/
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
"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.
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
Load and Configuration Hooks
Rails code can often be referenced on load of an application. Rails is responsible for the load order of these frameworks, so when you load frameworks, such as ActiveRecord::Base
, prematurely you are violating an implicit contract your application has with Rails. Moreover, by loading code such as ActiveRecord::Base
on boot of your application you are loading entire frameworks which may slow down your boot time and could cause conflicts with load order and boot of your application.
Load and configuration hooks are the API that allow you to hook into this initialization process without violating the load contract with Rails. This will also mitigate boot performance degradation and avoid conflicts.
Avoid loading Rails Frameworks
Since Ruby is a dynamic language, some code will cause different Rails frameworks to load. Take this snippet for instance:
ActiveRecord::Base.include(MyActiveRecordHelper)
This snippet means that when this file is loaded, it will encounter ActiveRecord::Base
. This encounter causes Ruby to look for the definition of that constant and will require it. This causes the entire Active Record framework to be loaded on boot.
ActiveSupport.on_load
is a mechanism that can be used to defer the loading of code until it is actually needed. The snippet above can be changed to:
ActiveSupport.on_load(:active_record) do
include MyActiveRecordHelper
end
This new snippet will only include MyActiveRecordHelper
when ActiveRecord::Base
is loaded.
When are Hooks called?
In the Rails framework these hooks are called when a specific library is loaded. For example, when ActionController::Base
is loaded, the :action_controller_base
hook is called. This means that all ActiveSupport.on_load
calls with :action_controller_base
hooks will be called in the context of ActionController::Base
(that means self
will be an ActionController::Base
).
Modifying Code to use Load Hooks
Modifying code is generally straightforward. If you have a line of code that refers to a Rails framework such as ActiveRecord::Base
you can wrap that code in a load hook.
Modifying calls to include
ActiveRecord::Base.include(MyActiveRecordHelper)
becomes
ActiveSupport.on_load(:active_record) do
# self refers to ActiveRecord::Base here,
# so we can call .include
include MyActiveRecordHelper
end
Modifying calls to prepend
ActionController::Base.prepend(MyActionControllerHelper)
becomes
ActiveSupport.on_load(:action_controller_base) do
# self refers to ActionController::Base here,
# so we can call .prepend
prepend MyActionControllerHelper
end
Modifying calls to class methods
ActiveRecord::Base.include_root_in_json = true
becomes
ActiveSupport.on_load(:active_record) do
# self refers to ActiveRecord::Base here
self.include_root_in_json = true
end
Available Load Hooks
These are the load hooks you can use in your own code. To hook into the initialization process of one of the following classes use the available hook.
Class | Hook |
---|---|
ActionCable |
action_cable |
ActionCable::Channel::Base |
action_cable_channel |
ActionCable::Connection::Base |
action_cable_connection |
ActionCable::Connection::TestCase |
action_cable_connection_test_case |
ActionController::API |
action_controller_api |
ActionController::API |
action_controller |
ActionController::Base |
action_controller_base |
ActionController::Base |
action_controller |
ActionController::TestCase |
action_controller_test_case |
ActionDispatch::IntegrationTest |
action_dispatch_integration_test |
ActionDispatch::Response |
action_dispatch_response |
ActionDispatch::Request |
action_dispatch_request |
ActionDispatch::SystemTestCase |
action_dispatch_system_test_case |
ActionMailbox::Base |
action_mailbox |
ActionMailbox::InboundEmail |
action_mailbox_inbound_email |
ActionMailbox::Record |
action_mailbox_record |
ActionMailbox::TestCase |
action_mailbox_test_case |
ActionMailer::Base |
action_mailer |
ActionMailer::TestCase |
action_mailer_test_case |
ActionText::Content |
action_text_content |
ActionText::Record |
action_text_record |
ActionText::RichText |
action_text_rich_text |
ActionView::Base |
action_view |
ActionView::TestCase |
action_view_test_case |
ActiveJob::Base |
active_job |
ActiveJob::TestCase |
active_job_test_case |
ActiveRecord::Base |
active_record |
ActiveStorage::Attachment |
active_storage_attachment |
ActiveStorage::Blob |
active_storage_blob |
ActiveStorage::Record |
active_storage_record |
ActiveSupport::TestCase |
active_support_test_case |
i18n |
i18n |
Available Configuration Hooks
Configuration hooks do not hook into any particular framework, but instead they run in context of the entire application.
Hook | Use Case |
---|---|
before_configuration |
First configurable block to run. Called before any initializers are run. |
before_initialize |
Second configurable block to run. Called before frameworks initialize. |
before_eager_load |
Third configurable block to run. Does not run if config.eager_load set to false. |
after_initialize |
Last configurable block to run. Called after frameworks initialize. |
Configuration hooks can be called in the Engine class.
module Blorgh
class Engine < ::Rails::Engine
config.before_configuration do
puts 'I am called before any initializers'
end
end
end