JSON rendering in Ruby on Rails - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the JSON output of this Rails controller action?

Consider this Rails controller action rendering JSON. What will be the exact JSON output sent to the client?

Ruby on Rails

def show
  user = User.new(id: 1, name: "Alice", admin: false)
  render json: { id: user.id, name: user.name }
end

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

B {"id":1,"name":"Alice","admin":false}

C {"id":1,"name":"Alice"}

D [{"id":1,"name":"Alice"}]

💡 Hint

Look at the hash passed to render json: and what keys it contains.

The render json: method converts the given hash to JSON. Only id and name keys are included, so the output JSON has only those fields.

📝 Syntax

intermediate

Which option correctly renders a JSON array of user names in Rails?

You want to render JSON with an array of user names from a list of User objects. Which code snippet produces the correct JSON output?

Ruby on Rails

users = [User.new(name: "Alice"), User.new(name: "Bob")]
render json: ???

A users.map { |u| {name: u.name} }

B users.pluck(:name)

C users.to_json(:only => [:name])

D users.map(&:name)

💡 Hint

You want a simple array of strings, not an array of hashes.

users.map(&:name) returns an array of names like ["Alice", "Bob"], which renders as a JSON array of strings.

🔧 Debug

advanced

Why does this Rails JSON render raise an error?

This code raises an error when rendering JSON. What is the cause?

Ruby on Rails

def index
  users = User.all
  render json: { users: users }
end

A ActiveRecord objects cannot be directly converted to JSON without serialization

B The hash keys must be strings, not symbols

C The users variable is nil causing a NoMethodError

D The render method requires a template file for JSON

💡 Hint

Think about how ActiveRecord objects convert to JSON by default.

ActiveRecord collections need to be serialized or converted to arrays/hashes before rendering JSON. Passing them directly can cause errors or unexpected output.

❓ state_output

advanced

What JSON output results from this Rails view with Jbuilder?

Given this Jbuilder template, what JSON will be rendered?

Ruby on Rails

json.array! @users do |user|
  json.id user.id
  json.name user.name
end

A {"users":[{"id":1,"name":"Alice"},{"id":2,"name":"Bob"}]}

B [{"id":1,"name":"Alice"},{"id":2,"name":"Bob"}]

C [{"user":{"id":1,"name":"Alice"}},{"user":{"id":2,"name":"Bob"}}]

D {"array":[{"id":1,"name":"Alice"},{"id":2,"name":"Bob"}]}

💡 Hint

array! creates a JSON array directly from the block.

json.array! builds a JSON array of objects with id and name keys for each user.

🧠 Conceptual

expert

Which option best explains how Rails handles JSON rendering with ActiveModel::Serializers?

When using ActiveModel::Serializers in Rails, how does the JSON rendering process work?

A The serializer defines attributes and associations to include, and Rails uses it to build JSON instead of default to_json

B Rails automatically converts all model attributes to JSON without any customization

C ActiveModel::Serializers requires manual JSON string construction in the controller

D Serializers replace the controller render method and output HTML instead of JSON

💡 Hint

Think about how serializers customize JSON output.

ActiveModel::Serializers lets you specify exactly which attributes and relationships to include in JSON. Rails uses the serializer instead of default to_json.