Before and after filters in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Rails controller with before_action?

Consider this Rails controller snippet. What will be printed when the show action is called?

Ruby on Rails

class ArticlesController < ApplicationController
  before_action :set_article, only: [:show]

  def show
    render plain: @article.title
  end

  private

  def set_article
    @article = OpenStruct.new(title: "Hello World")
  end
end

A nil

B Raises NoMethodError

C "" (empty string)

D "Hello World"

💡 Hint

Remember that before_action runs the method before the action method.

The before_action :set_article runs before show, so @article is set. The show action renders the title "Hello World".

❓ lifecycle

intermediate

Which callback runs after the action method in Rails controllers?

Given these Rails controller callbacks, which one runs after the action method?

Ruby on Rails

class UsersController < ApplicationController
  before_action :authenticate_user
  after_action :log_action

  def index
    render plain: "User list"
  end

  private

  def authenticate_user
    # authentication logic
  end

  def log_action
    # logging logic
  end
end

A before_action :authenticate_user

B around_action :wrap_action

C after_action :log_action

D skip_before_action :authenticate_user

💡 Hint

Think about which callback runs after the controller action finishes.

The after_action callback runs after the action method completes, so log_action runs after index.

📝 Syntax

advanced

What error does this Rails filter code raise?

Examine this Rails controller code. What error will it raise when loaded?

Ruby on Rails

class ProductsController < ApplicationController
  before_action :set_product, only: [:show, :edit]

  def show
    render plain: @product.name
  end

  private

  def set_product
    @product = Product.find(params[:id])
  end
end

A No error, code runs fine

B NoMethodError: undefined method `find' for nil:NilClass

C ArgumentError: wrong number of arguments (given 0, expected 1)

D NameError: undefined local variable or method `params' for ProductsController

💡 Hint

Consider how params is available in controller methods.

The params method is available in controller instance methods, so set_product can call Product.find(params[:id]) without error.

🔧 Debug

advanced

Does this after_action run when the action redirects?

In this Rails controller, will the after_action callback run?

Ruby on Rails

class OrdersController < ApplicationController
  after_action :clear_cart, only: [:create]

  def create
    # order creation logic
    redirect_to order_path(1)
  end

  private

  def clear_cart
    session[:cart] = nil
  end
end

A Yes, the after_action runs even if the action redirects

B No, the method clear_cart is private and cannot be called as a callback

C No, the after_action is set for :create but the action name is misspelled

D No, after_action callbacks do not run if the action redirects

💡 Hint

Redirects do not prevent after_action callbacks from running.

after_action callbacks are executed after the action method completes execution, regardless of whether the action performed a render or a redirect.

🧠 Conceptual

expert

What is the order of execution for these Rails controller callbacks?

Given these callbacks in a Rails controller, what is the correct order they run when an action is called?

Ruby on Rails

class CommentsController < ApplicationController
  before_action :authenticate_user
  around_action :wrap_in_transaction
  after_action :log_comment

  def create
    # create comment logic
    render plain: "Created"
  end

  private

  def authenticate_user
    # authentication
  end

  def wrap_in_transaction
    ActiveRecord::Base.transaction do
      yield
    end
  end

  def log_comment
    # logging
  end
end

A 2,1,3,5,4

B 1,2,3,4,5

C 1,3,2,4,5

D 2,3,1,4,5

💡 Hint

Remember that around_action wraps the action and other callbacks.

The before_action runs first, then the around_action starts and yields to the action, then the around_action ends, and finally the after_action runs.