JSON rendering in Ruby on Rails - Step-by-Step Execution

Concept Flow - JSON rendering

Controller receives request

↓

Prepare data in controller

↓

Call render json: data

↓

Rails converts data to JSON

↓

Send JSON response to client

Rails controller gets a request, prepares data, renders it as JSON, and sends it back to the client.

Execution Sample

Ruby on Rails

def show
  user = User.find(params[:id])
  render json: user
end

This code finds a user by id and sends the user data as JSON response.

Execution Table

Step	Action	Data State	Result
1	Receive request with user id	params[:id] = 3	Ready to find user
2	Find user by id	user = User object with id 3	User data loaded
3	Call render json: user	user object	Rails converts user to JSON string
4	Send JSON response	JSON string of user	Client receives JSON data

💡 Response sent, request cycle ends

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
params[:id]	nil	3	3	3
user	nil	{id:3, name:"Alice"}	{id:3, name:"Alice"}	{id:3, name:"Alice"}
JSON output	nil	nil	{"id":3,"name":"Alice"}	{"id":3,"name":"Alice"}

Key Moments - 2 Insights

Why do we use render json: user instead of just render user?

render json: user tells Rails to convert the user object into JSON format before sending. Just render user tries to find a template named 'user', which is different. See execution_table step 3.

What happens if the user is not found?

If User.find fails, Rails raises an error and does not reach render json. You need error handling to manage this case, which is not shown here.

Visual Quiz - 3 Questions

Look at the execution_table, what is the data state after step 2?

A params[:id] is 3 and user is loaded as User object

B user is a JSON string

C Rails has sent the response

D params[:id] is nil

💡 Hint

Check the 'Data State' column in row for step 2

At which step does Rails convert the user object to JSON?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' and 'Result' columns in execution_table

If params[:id] was missing, what would happen in this flow?

A User is found with id nil

B Rails raises an error before rendering JSON

C Rails sends empty JSON response

D Rails renders a template instead

💡 Hint

Refer to key_moments about missing user or id

Concept Snapshot

Rails JSON rendering:
- Use render json: object in controller
- Rails converts object to JSON string
- Sends JSON response to client
- Handles hashes, arrays, ActiveRecord objects
- Errors if object not found or invalid
- Common for API responses

Full Transcript

In Rails, when a controller receives a request, it prepares data such as finding a user by id. Then it calls render json: user, which tells Rails to convert the user object into a JSON string. Rails sends this JSON string as the response to the client. If the user is not found, Rails raises an error before rendering. This flow helps build APIs that send data in JSON format.

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