JSON rendering in Ruby on Rails - Performance & Optimization

Performance: JSON rendering

MEDIUM IMPACT

This affects server response time and client load speed by controlling how JSON data is generated and sent.

Rendering JSON response for an API endpoint

Ruby on Rails

render json: @users.as_json(only: [:id, :name], include: { posts: { only: [:id, :title] } })

Limits fields and avoids expensive method calls, reducing CPU and payload size.

📈 Performance Gainresponse time cut by 70%, payload size reduced by 40%

Rendering JSON response for an API endpoint

Ruby on Rails

render json: @users.to_json(include: :posts, methods: :expensive_method)

Serializing large nested associations and calling expensive methods on each record causes slow response and high CPU usage.

📉 Performance Costblocks rendering for 200ms+ on large datasets, increases payload size by 50%+

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Rendering large nested JSON with methods	N/A (server-side)	N/A	N/A	[X] Bad
Rendering limited JSON fields with includes	N/A (server-side)	N/A	N/A	[OK] Good

Rendering Pipeline

Rails converts Ruby objects to JSON strings on the server, then sends them over HTTP to the browser, which parses and renders the data.

→Server CPU processing

→Network transfer

→Browser JSON parsing

⚠️ BottleneckServer CPU processing during serialization

Core Web Vital Affected

LCP

This affects server response time and client load speed by controlling how JSON data is generated and sent.

Optimization Tips

1Avoid serializing large nested associations without limits.

2Use as_json with only and include to control output size.

3Paginate large datasets to reduce server load and response size.

Performance Quiz - 3 Questions

What is a common cause of slow JSON rendering in Rails APIs?

A Sending JSON responses without pagination

B Using simple to_json with limited fields

C Serializing large nested associations with expensive method calls

D Caching JSON responses

DevTools: Network

How to check: Open DevTools, go to Network tab, filter XHR requests, select the JSON response, and check its size and timing.

What to look for: Look for large payload sizes and long waiting times indicating slow JSON rendering or large data transfer.

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