API-only application setup in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to create an API-only Rails application?

It means setting up a Rails app that only serves data through APIs, without rendering HTML views or using frontend features like layouts or assets.

beginner

Which command creates a new Rails app configured as API-only?

rails new myapp --api

intermediate

Why does an API-only Rails app skip middleware like session or cookies by default?

Because APIs usually don't need browser sessions or cookies, so skipping them makes the app lighter and faster.

intermediate

How do you add middleware back if needed in an API-only Rails app?

You can add middleware manually in config/application.rb using config.middleware.use followed by the middleware class.

beginner

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

JSON is the default response format, as APIs usually send data in JSON to clients.

Which flag creates a Rails app optimized for APIs only?

A --no-assets

B --skip-active-record

C --minimal

D --api

The --api flag sets up Rails with API-only defaults.

What is NOT included by default in an API-only Rails app?

A Session middleware

B JSON rendering

C Routing

D Controllers

Session middleware is skipped by default in API-only apps.

How do API-only Rails apps usually send data to clients?

A XML files

B HTML pages

C JSON responses

D Plain text files

JSON is the common format for API data exchange.

If you want to add cookie support in an API-only Rails app, what should you do?

A Recreate app without --api

B Add cookie middleware manually

C Use --full-stack flag

D Cookies are always enabled

You add middleware manually in config/application.rb.

Which of these is a benefit of API-only Rails apps?

A Faster response times

B Built-in HTML templates

C Automatic asset compilation

D Full browser session support

Skipping unneeded features makes API apps faster.

Explain how to create and configure a new API-only Rails application.

Think about the command and what changes in app behavior.

Describe how middleware works differently in API-only Rails apps compared to full-stack apps.

Focus on what middleware is included or excluded.

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