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.

Practice

1. What is the main purpose of a before_action filter in a Rails controller?

easy

A. To run code before a controller action executes

B. To run code after a controller action executes

C. To skip a controller action entirely

D. To define a new controller action

Solution

1. Step 1: Understand the role of filters

   Filters in Rails run code around controller actions to modify behavior.

2. Step 2: Identify the timing of before_action

   before_action runs code before the main action method executes.

3. Final Answer:

   To run code before a controller action executes -> Option A

4. Quick Check:

   before_action = runs before action [OK]

Hint: Before filters run code before actions start [OK]

Common Mistakes:

• Confusing before_action with after_action
• Thinking before_action skips actions
• Assuming before_action defines new actions

2. Which of the following is the correct syntax to define an after filter named log_action in a Rails controller?

easy

A. after_filter :log_action

B. after :log_action

C. before_action :log_action

D. after_action :log_action

Solution

1. Step 1: Recall the modern syntax for after filters

   Rails uses after_action to define code that runs after controller actions.

2. Step 2: Check each option

   after_filter :log_action uses deprecated after_filter. after_action :log_action uses correct after_action. before_action :log_action is for before filters. after :log_action is invalid syntax.

3. Final Answer:

   after_action :log_action -> Option D

4. Quick Check:

   Use after_action for after filters [OK]

Hint: Use after_action, not after_filter, for after filters [OK]

Common Mistakes:

• Using deprecated after_filter instead of after_action
• Mixing before_action with after filters
• Incorrect syntax like 'after :method'

3. Given this controller code:

class PostsController < ApplicationController
  before_action :set_post
  after_action :log_post

  def show
    render plain: @post.title
  end

  private
  def set_post
    @post = Post.find(params[:id])
  end

  def log_post
    Rails.logger.info "Post viewed: #{@post.id}"
  end
end

What will be the output when GET /posts/5 is called, assuming the post with ID 5 has title "Hello"?

medium

A. The page shows "Hello" and logs "Post viewed: 5" after rendering

B. The page shows "Hello" but does not log anything

C. The page shows nothing and logs "Post viewed: 5"

D. An error occurs because @post is not set

Solution

1. Step 1: Understand filter order and effects

   before_action :set_post runs before show, setting @post. after_action :log_post runs after show completes.

2. Step 2: Trace the request flow

   When show runs, @post is set, so render plain: @post.title outputs "Hello". After rendering, log_post logs "Post viewed: 5".

3. Final Answer:

   The page shows "Hello" and logs "Post viewed: 5" after rendering -> Option A

4. Quick Check:

   before_action sets data, after_action logs after render [OK]

Hint: before_action sets data, after_action runs after response [OK]

Common Mistakes:

• Thinking after_action runs before rendering
• Assuming @post is nil in show
• Confusing logging output with page content

4. Identify the error in this controller code:

class UsersController < ApplicationController
  before_action :authenticate_user
  after_action :clear_session

  def index
    # list users
  end

  private
  def authenticate_user
    redirect_to login_path unless current_user
  end

  def clear_session
    session[:temp] = nil
  end
end

medium

A. The methods authenticate_user and clear_session must be public

B. The before_action should be after_action for authentication

C. The after_action runs even if redirect happens, causing errors

D. There is no error; the code works as expected

Solution

1. Step 1: Analyze filter behavior with redirects

   If authenticate_user redirects, the action does not run, but after_action still runs.

2. Step 2: Understand consequences of after_action after redirect

   Running clear_session after redirect can cause unexpected side effects or errors.

3. Final Answer:

   The after_action runs even if redirect happens, causing errors -> Option C

4. Quick Check:

   after_action runs even after redirect [OK]

Hint: after_action runs even if before_action redirects [OK]

Common Mistakes:

• Assuming after_action skips on redirect
• Thinking authentication should be after_action
• Believing private methods must be public

5. You want to run a before_action filter named check_admin only before the edit and update actions in a Rails controller. Which is the correct way to write this?

hard

A. before_action :check_admin, except: [:edit, :update]

B. before_action :check_admin, only: [:edit, :update]

C. after_action :check_admin, only: [:edit, :update]

D. before_filter :check_admin, only: [:edit, :update]

Solution

1. Step 1: Identify correct filter type and options

   Use before_action to run code before actions. Use only: to limit to specific actions.

2. Step 2: Evaluate each option

   before_action :check_admin, only: [:edit, :update] correctly uses before_action with only: [:edit, :update]. before_action :check_admin, except: [:edit, :update] uses except, which excludes those actions. after_action :check_admin, only: [:edit, :update] uses after_action, wrong timing. before_filter :check_admin, only: [:edit, :update] uses deprecated before_filter.

3. Final Answer:

   before_action :check_admin, only: [:edit, :update] -> Option B

4. Quick Check:

   Use before_action with only: for specific actions [OK]

Hint: Use before_action with only: to target specific actions [OK]

Common Mistakes:

• Using except instead of only
• Using after_action instead of before_action
• Using deprecated before_filter