JSON rendering in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is JSON rendering in Rails?

JSON rendering in Rails means sending data from the server to the client in JSON format, which is easy for web apps to use and understand.

beginner

How do you render JSON in a Rails controller?

Use render json: @object inside a controller action to send the object as JSON to the client.

intermediate

What does render json: @user, only: [:id, :name] do?

It sends only the id and name fields of the @user object as JSON, hiding other data.

beginner

Why use JSON rendering instead of HTML in Rails APIs?

JSON is lightweight and easy for apps and JavaScript to read, making it perfect for APIs that send data, not full web pages.

intermediate

How can you customize JSON output in Rails?

You can use options like only, except, or methods in render json: or create custom serializers.

Which Rails method sends data as JSON to the client?

A redirect_to @data

B render json: @data

C send_data @data

D render html: @data

Use render json: @data to send JSON data from a Rails controller.

How do you exclude fields from JSON output in Rails?

A render json: @obj, only: [:field1]

B render json: @obj, hide: [:field4]

C render json: @obj, except: [:field2]

D render json: @obj, include: [:field3]

The except option excludes specified fields from the JSON output.

What format does Rails use when you write render json: @user?

A JSON

B HTML

C XML

D Plain text

The render json: method sends data in JSON format.

Which option adds extra methods to JSON output in Rails?

A methods

B only

C except

D include

The methods option adds custom methods to the JSON output.

Why is JSON preferred for APIs over HTML?

A JSON cannot be parsed by JavaScript

B JSON is heavier than HTML

C HTML is not supported by browsers

D JSON is easier for apps to read and use

JSON is lightweight and easy for apps and JavaScript to read, making it ideal for APIs.

Explain how to render JSON in a Rails controller and how to control which fields are included.

Think about how you send data and filter fields.

Describe why JSON rendering is important for building APIs in Rails.

Consider how apps use data from servers.

Practice

1. What does render json: @user do in a Rails controller?

easy

A. It sends the @user object as a JSON response to the client.

B. It renders an HTML view named 'json'.

C. It saves the @user object to the database in JSON format.

D. It redirects the user to a JSON page.

Solution

1. Step 1: Understand the render method with json option

   The render json: command in Rails sends data as JSON format to the client instead of rendering an HTML view.

2. Step 2: Identify the effect of @user in this context

   Passing @user sends the user object data as JSON in the HTTP response.

3. Final Answer:

   It sends the @user object as a JSON response to the client. -> Option A

4. Quick Check:

   render json: @user = send JSON response [OK]

Hint: render json: sends data as JSON response [OK]

Common Mistakes:

• Thinking it renders an HTML view named 'json'
• Confusing render json with saving data
• Assuming it redirects to another page

2. Which of the following is the correct syntax to render a JSON response with a status code 201 in Rails?

easy

A. render json: @post, code: 201

B. render json => @post, status = 201

C. render json: @post status 201

D. render json: @post, status: 201

Solution

1. Step 1: Identify correct Ruby hash syntax for render options

   In Rails, options are passed as key: value pairs separated by commas, e.g., json: @post, status: 201.

2. Step 2: Check each option for syntax correctness

   render json: @post, status: 201 uses correct Ruby hash syntax. Options B, C, and D have syntax errors or wrong keys.

3. Final Answer:

   render json: @post, status: 201 -> Option D

4. Quick Check:

   Correct Ruby hash syntax = render json: @post, status: 201 [OK]

Hint: Use commas and colons for options in render [OK]

Common Mistakes:

• Using => instead of : in Ruby 3+ syntax
• Missing commas between options
• Using wrong keys like code instead of status

3. Given the controller code:

def show
  @user = User.find(1)
  render json: @user.to_json(only: [:id, :name])
end

What will the JSON response include?

medium

A. {"id":1,"name":"Alice"}

B. {"name":"Alice"}

C. {"id":1}

D. {"id":1,"name":"Alice","email":"[e-mail]"}

Solution

1. Step 1: Understand the use of to_json(only: [:id, :name])

   This limits the JSON output to only the id and name fields of the user object.

2. Step 2: Identify which fields appear in the JSON response

   Only id and name are included, so email is excluded.

3. Final Answer:

   {"id":1,"name":"Alice"} -> Option A

4. Quick Check:

   only: [:id, :name] = id and name fields only [OK]

Hint: only: limits JSON fields included [OK]

Common Mistakes:

• Assuming all fields are included by default
• Confusing only: with except:
• Not calling to_json with options

4. What is wrong with this Rails controller code?

def index
  @users = User.all
  render json: @users only: [:id, :email]
end

medium

A. The @users variable should be a single user, not all users.

B. The only option should be inside to_json method, not passed directly to render.

C. You cannot render collections as JSON in Rails.

D. The render method requires a template name.

Solution

1. Step 1: Check how to limit JSON fields for collections

   The only option must be passed inside to_json or as a render json: option hash, not as a separate argument.

2. Step 2: Identify the correct way to pass only option

   Correct usage: render json: @users.to_json(only: [:id, :email]) or render json: @users, only: [:id, :email] inside a hash, but the given code passes only outside the hash.

3. Final Answer:

   The only option should be inside to_json method, not passed directly to render. -> Option B

4. Quick Check:

   only option must be inside to_json or render json hash [OK]

Hint: Pass only inside to_json or render json hash [OK]

Common Mistakes:

• Passing only as separate argument outside render hash
• Thinking collections can't be rendered as JSON
• Forgetting to call to_json with options

5. You want to render a JSON response with a list of users, but only include users with active status true and only their id and name fields. Which code correctly does this in a Rails controller?

hard

A. render json: User.where(active: true) only: [:id, :name]

B. render json: User.all.select(:id, :name).where(active true)

C. render json: User.where(active: true).to_json(only: [:id, :name])

D. render json: User.where(active: true).only(:id, :name)

Solution

1. Step 1: Filter users by active status

   Use User.where(active: true) to get only active users.

2. Step 2: Limit JSON fields to id and name

   Call to_json(only: [:id, :name]) to include only those fields in the JSON output.

3. Step 3: Check each option for correctness

   render json: User.where(active: true).to_json(only: [:id, :name]) correctly chains filtering and JSON field limiting. render json: User.all.select(:id, :name).where(active: true) has wrong method order and will error. render json: User.where(active: true), only: [:id, :name] passes only incorrectly. render json: User.where(active: true).only(:id, :name) uses a non-existent only method on ActiveRecord relation.

4. Final Answer:

   render json: User.where(active: true).to_json(only: [:id, :name]) -> Option C

5. Quick Check:

   Filter then to_json with only: fields [OK]

Hint: Filter first, then call to_json with only: [OK]

Common Mistakes:

• Using only as a method on ActiveRecord relation
• Passing only outside to_json or render json hash
• Selecting fields before filtering causing errors