API-only application setup in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the default middleware stack in a Rails API-only app?

In a Rails API-only application, which middleware is excluded by default compared to a full Rails app?

A Request ID middleware is excluded by default.

B Static file server middleware is excluded by default.

C Session and cookie middleware are excluded by default.

D Logger middleware is excluded by default.

💡 Hint

Think about what an API-only app does not need compared to a full web app.

API-only apps do not use sessions or cookies by default, so their middleware is excluded. Other middleware like logging and request ID remain.

📝 Syntax

intermediate

How to generate a new Rails API-only app?

Which command correctly creates a new Rails API-only application named my_api?

A rails new my_api --only-api

B rails new my_api --api

C rails new my_api --api-only

D rails new my_api --api_mode

💡 Hint

The flag is short and starts with two dashes.

The correct flag to create an API-only Rails app is --api.

❓ state_output

advanced

What is the default response format in Rails API-only controllers?

Given a Rails API-only controller without explicit format settings, what is the default response format when a client requests data?

Ruby on Rails

class BooksController < ApplicationController
  def index
    render json: { books: ['Ruby', 'Rails'] }
  end
end

A Plain text

B XML

C HTML

D JSON

💡 Hint

API-only apps focus on data exchange formats.

Rails API-only apps default to JSON responses for data rendering.

🔧 Debug

advanced

Why does this API-only app raise a Missing CSRF Token error?

Consider this Rails API-only app controller code:

class PostsController < ApplicationController
  protect_from_forgery with: :exception

  def create
    render json: { status: 'ok' }
  end
end

Why does this cause a CSRF token missing error when posting?

Ruby on Rails

class PostsController < ApplicationController
  protect_from_forgery with: :exception

  def create
    render json: { status: 'ok' }
  end
end

A API-only apps disable CSRF protection by default; enabling it causes errors without tokens.

B The controller is missing a before_action to authenticate the user.

C The render json call is incorrect and causes the error.

D The routes file is missing the POST route for create.

💡 Hint

Think about what Rails disables by default in API-only mode.

CSRF protection is disabled by default in API-only apps because APIs usually use tokens or other auth methods. Enabling it without tokens causes errors.

🧠 Conceptual

expert

What is the main reason to choose API-only mode in Rails?

Why would a developer choose to create a Rails application with the --api flag instead of a full Rails app?

A To build a lightweight backend focused on JSON APIs without views or assets.

B To enable automatic generation of HTML views and layouts.

C To create a full-stack app with integrated frontend templates.

D To include all default middleware for session and cookie management.

💡 Hint

Think about what an API-only app does NOT include.

API-only mode creates a slim backend focused on serving JSON data, skipping views, assets, and session middleware.

Practice

1. What is the main purpose of creating a Rails application with the --api flag?

easy

A. To generate a full Rails app with HTML views and assets

B. To enable automatic frontend scaffolding

C. To create a lightweight backend that serves JSON responses only

D. To create a desktop application using Rails

Solution

1. Step 1: Understand the --api flag purpose

   The --api flag creates a Rails app optimized for API use, focusing on JSON responses without HTML views.

2. Step 2: Compare with full Rails app

   Full Rails apps include views and assets, but API-only apps skip these for lightweight backend.

3. Final Answer:

   To create a lightweight backend that serves JSON responses only -> Option C

4. Quick Check:

   API-only app = JSON backend [OK]

Hint: API-only means JSON backend, no HTML views [OK]

Common Mistakes:

• Thinking API-only apps generate HTML views
• Confusing API-only with full Rails app
• Assuming API-only creates frontend code

2. Which command correctly creates a new Rails API-only application named my_api?

easy

A. rails new my_api --full

B. rails new my_api --api

C. rails generate api my_api

D. rails create my_api --api-only

Solution

1. Step 1: Recall the Rails new command syntax

   The correct way to create an API-only app is using rails new app_name --api.

2. Step 2: Match the command with the app name

   Using rails new my_api --api matches the requirement exactly.

3. Final Answer:

   rails new my_api --api -> Option B

4. Quick Check:

   rails new + app name + --api = correct command [OK]

Hint: Use 'rails new app_name --api' to create API apps [OK]

Common Mistakes:

• Using --full instead of --api
• Using 'generate' instead of 'new'
• Incorrect flag like --api-only

3. Given this Rails API-only controller code, what will be the JSON response when accessing /api/greetings?

class Api::GreetingsController < ApplicationController
  def index
    render json: { message: 'Hello, API!' }
  end
end

medium

A. Redirect to root path

B. Hello, API!

C. Error: Missing view template

D. {"message":"Hello, API!"}

Solution

1. Step 1: Analyze the controller action

   The index action uses render json: to send a JSON response with a message.

2. Step 2: Understand API-only behavior

   API-only apps respond with JSON, so the output is the JSON string of the hash.

3. Final Answer:

   {"message":"Hello, API!"} -> Option D

4. Quick Check:

   render json = JSON output [OK]

Hint: render json sends JSON, not HTML [OK]

Common Mistakes:

• Expecting HTML output in API-only app
• Assuming missing view causes error
• Thinking it redirects automatically

4. You created a Rails API-only app but get an error: ActionController::UnknownFormat when requesting JSON. What is the likely cause?

medium

A. The controller action does not specify render json:

B. The app was created without the --api flag

C. The route is missing in routes.rb

D. The database is not migrated

Solution

1. Step 1: Understand the error meaning

   ActionController::UnknownFormat means the controller does not know how to respond to the requested format.

2. Step 2: Check controller response method

   If the action lacks render json:, Rails can't respond with JSON, causing this error.

3. Final Answer:

   The controller action does not specify render json: -> Option A

4. Quick Check:

   UnknownFormat = missing render json [OK]

Hint: Always use render json in API controllers [OK]

Common Mistakes:

• Assuming missing --api flag causes this error
• Thinking missing route causes UnknownFormat
• Blaming database migration for format errors

5. You want to add authentication to your Rails API-only app using token-based headers. Which setup step is essential to support this in an API-only app?

hard

A. Add middleware for session and cookies support manually

B. Generate views for login and logout pages

C. Use rails new without --api to enable sessions

D. Disable JSON responses to allow HTML forms

Solution

1. Step 1: Understand API-only app default middleware

   API-only apps exclude middleware for sessions and cookies by default to stay lightweight.

2. Step 2: Add middleware manually for token auth

   To support token-based authentication using headers, you often need session or cookie middleware added manually.

3. Final Answer:

   Add middleware for session and cookies support manually -> Option A

4. Quick Check:

   API-only needs manual middleware for sessions [OK]

Hint: Add session middleware manually for token auth [OK]

Common Mistakes:

• Expecting views for login in API-only app
• Recreating full app without --api flag
• Disabling JSON responses breaks API design