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Bringing real OOP to Rails controllers.
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Focused Controller: Bringing Real OOP to Rails Controllers

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Classical Rails controllers violate the Single Responsibility Principle.

Each different "action" has separate responsibilities: A create action does something entirely different to a destroy action, yet they end up lumped into the same object.

This has two unfortunate side effects:

  1. We end up using instance variables to share data with our views when we should really be using methods. Using instance variables for this purpose breaks encapsulation and can lead to subtle bugs. For example, an undeclared instance variable in a view will be nil, rather than raising an error when referenced.

  2. We misuse before_filters to share functionality between actions. Instead of using proper OO patterns like inheritance and mixins to keep our code DRY, we shoe-horn before_filter with :only or :except to share chunks of code between specific actions.

A related problem with controllers is that the way we test them is slow and esoteric. Rather than calling a single method on our controller object and making assertions about what happened, we generate a request to put through the full controller stack, exercising large amounts of internal Rails code in each test case. These are not unit tests, yet they are often used to test logic.

Focused Controller aims to address these issues by using a single, focused object for each action. These object should have only one responsibility and be straightforward to instantiate and test in isolation.

Feedback needed

This project is in early stages, and while I have been using it successfully on a production application, I'm very keen for others to start experimenting with it and providing feedback.

Note that I will follow SemVer, and the project is currently pre-1.0, so there could be API changes. However if the user base grows significantly, then I will try to avoid painful changes.

There is a mailing list for discussion.


Focused Controller changes Rails' conventions. Rather than controllers being classes that contain one method per action, controllers are now namespaces and each action is a class within that namespace.

Controllers which wish to use this convention include the FocusedController::Mixin module. This means you can start using Focused Controller in an existing project without having to rewrite all your existing controller code.

An example:

module PostsController
  # Action is a common superclass for all the actions
  # inside `PostsController`.
  class Action < ApplicationController
    include FocusedController::Mixin

  class Index < Action
    def run
      # Your code here.

    # No instance variables are shared with the view. Instead,
    # public methods are defined.
    def posts
      @posts ||= Post.all

    # To prevent yourself having to write `controller.posts`
    # in the view, you can declare the method as a helper
    # method which means that calling `posts` automatically
    # delegates to the controller.
    helper_method :posts

  # Actions do not need to declare a `run` method - the default
  # implementation inherited from `FocusedController::Mixin` is an
  # empty method.
  class Show < Action
    # Here's a shorter way to declare a method that is also a
    # helper_method
    expose(:post) { Post.find params[:id] }

    # You can also call expose without a block, in which case an
    # attr_reader and a helper_method are declared
    expose :first_comment


Rails' normal routing assumes your actions are methods inside an object whose name ends with 'controller'. For example:

get '/posts/new' => 'posts#new'

will route GET /posts/new to PostsController#new.

To get around this, we use the focused_controller_routes helper:

focused_controller_routes do
  get '/posts/new' => 'posts#new'

The route will now map to PostsController::New#run.

This is similar to writing:

get '/posts/new' => proc { |env| }

All the normal routing macros are also supported:

focused_controller_routes do
  resources :posts

Functional Testing

Though it's not encouraged, focused controllers can be tested in the classical 'functional' style. This can be a useful interim measure when converting a controller to be properly unit tested.

It no longer makes sense to specify the action name to be called as the action name is always "run". So this is omitted:

module UsersController
  class CreateTest < ActionController::TestCase
    include FocusedController::FunctionalTestHelper

    test "should create user" do
      assert_difference('User.count') do
        post user: { name: 'Jon' }

      assert_redirected_to user_path(@controller.user)

There is also an equivalent helper for RSpec:

describe UsersController do
  include FocusedController::RSpecFunctionalHelper

  describe UsersController::Create do
    it "should create user" do
      expect { post user: { name: 'Jon' } }.to change(User, :count).by(1)
      response.should redirect_to(user_path(subject.user))

Don't forget to add the line below to your spec_helper.rb file:

require 'focused_controller/rspec_functional_helper'

Unit Testing

A better way to test your controllers is with unit tests. This involves creating an instance of your action class and calling methods on it. For example, to test that your user method finds the correct user, you might write:

module UsersController
  class ShowTest < ActiveSupport::TestCase
    test 'finds the user' do
      user = User.create

      controller =
      controller.params = { id: }

      assert_equal user, controller.user

The #run method

Testing the code in your #run method is a little more involved, depending on what's in it. For example, your #run method may use (explicitly or implicitly) any of the following objects:

  • request
  • response
  • params
  • session
  • flash
  • cookies

To make the experience smoother, Focused Controller sets up mock versions of these objects, much like with classical functional testing. It also provides accessors for these objects in your test class.

The fact that we have to do this is an indication of high coupling between the controller and these other objects. In the future, I want to look at ways to reduce this coupling and make testing more straightforward and obvious.

In the mean time, here is an example:

module UsersController
  class CreateTest < ActiveSupport::TestCase
    include FocusedController::TestHelper

    test "should create user" do
      assert_difference('User.count') do
        req user: { name: 'Jon' }

      assert_redirected_to user_path(controller.user)

The req helper

The req method runs the "request", but it does not go through the Rack stack. It simply sets up the params, session, flash, and then calls the #run method. The following are equivalent:

req({ x: 'x' }, { y: 'y' }, { z: 'z' })
controller.params = { x: 'x' }
session.update(y: 'y')
flash.update(z: 'z')


You also have access to the normal assertions found in Rails' functional tests:

  • assert_template
  • assert_response
  • assert_redirected_to

However, I intend to consider alternatives to these. For example,

assert_equal users_path, controller.location

seems lot more straightforward and explicit to me than:

assert_redirected_to users_path


We're not testing through the Rack stack. We're just calling the #run method. Therefore, filters do not get run. This is a feature: if your filter code is truly orthogonal to your controller code it should be unit tested separately. If it is not orthogonal then you should find a way to invoke it more explicitly than via filters.

(At this point I will ask: if it is truly orthogonal, why not make it a Rack middleware?)


There is a helper for RSpec as well:

describe UsersController do
  include FocusedController::RSpecHelper

  describe UsersController::Create do
    test "should create user" do
      expect { req user: { name: 'Jon' } }.to change(User, :count).by(1)
      response.should redirect_to(user_path(subject.user))

Don't forget to add the line below to your spec_helper.rb file:

require 'focused_controller/rspec_helper'

Isolated unit tests

It is possible to completely decouple your focused controller tests from the Rails application. This means you don't have to pay the penalty of starting up Rails every time you want to run a test. The benefit this brings will depend on how coupled your controllers/tests are to other dependencies.

Your config/routes.rb file is a dependency. When you use a URL helper you are depending on that file. As this is a common dependency, Focused Controller provides a way to stub out URL helpers:

module UsersController
  class CreateTest < ActiveSupport::TestCase
    include FocusedController::TestHelper
    stub_url :user

    # ...

The stub_url declaration will make the user_path and user_url methods in your test and your controller return stub objects. These can be compared, so user_path(user1) == user_path(user1), but user_path(user1) != user_path(user2).

Speed comparison

Here's a comparison of running the same test in each of the different styles:


  • Test time: 0.154842s, 45.2075 tests/s, 64.5821 assertions/s
  • Total time: 3.380s


  • Test time: 0.046101s, 151.8393 tests/s, 216.9133 assertions/s
  • Total time: 3.578s

Isolated Unit

  • Test time: 0.016669s, 419.9434 tests/s, 599.9191 assertions/s
  • Total time: 2.398s

More examples

The acceptance tests for Focused Controller exercise a complete Rails application, which uses the plugin. Therefore, you might wish to look there to get more of an idea about how it can be used.

(Note that the code there is based on Rails' scaffolding, not how I would typically write controllers and tests, necessarily.)

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