Ruby on Rails is one of the most popular web development frameworks, known for its efficiency, scalability, and developer-friendly syntax. Whether you’re a beginner or an experienced developer preparing for a job interview, mastering Rails concepts is crucial.

In this guide, we’ve compiled the top 100 Ruby on Rails interview questions with detailed answers and real-world examples. These questions cover ActiveRecord, MVC architecture, routing, caching, background jobs, API development, security, performance optimization, and more. By the end of this post, you’ll be well-prepared to tackle any Rails-related interview confidently.

Let’s dive in! 🚀

1. What is Ruby on Rails?

Answer:
Ruby on Rails (Rails) is a web application framework written in Ruby. It follows the Model-View-Controller (MVC) pattern and emphasizes convention over configuration (CoC) and Don’t Repeat Yourself (DRY) principles.

Example:

# Simple Rails Controller
class UsersController < ApplicationController
  def index
    @users = User.all
  end
end

2. What is MVC in Rails?

Answer:
MVC stands for Model, View, Controller:

• Model: Handles database logic (ActiveRecord).
• View: Displays data (ERB templates).
• Controller: Manages requests & responses.

3. What is Active Record in Rails?

Answer:
Active Record is the ORM (Object Relational Mapping) layer in Rails that simplifies database interactions.

Example:

class User < ApplicationRecord
end

# Fetch all users
User.all

4. What are migrations in Rails?

Answer:
Migrations allow database schema changes using Ruby instead of raw SQL.

Example:

rails generate migration AddEmailToUsers email:string

5. What is the difference between delete and destroy in Rails?

Answer:

• delete: Removes a record without callbacks.
• destroy: Removes a record and runs callbacks.

Example:

user = User.find(1)
user.delete   # No callbacks
user.destroy  # Runs callbacks

6. What is the Rails Asset Pipeline?

Answer:
It compiles and serves assets (CSS, JS) efficiently.

7. What are Strong Parameters in Rails?

Answer:
They prevent mass-assignment vulnerabilities by allowing only permitted attributes.

Example:

params.require(:user).permit(:name, :email)

8. Explain the difference between has_one and belongs_to.

Answer:

• has_one: A parent has one child.
• belongs_to: A child belongs to a parent.

Example:

class User < ApplicationRecord
  has_one :profile
end

class Profile < ApplicationRecord
  belongs_to :user
end

9. What are Callbacks in Rails?

Answer:
Callbacks are lifecycle methods that run at certain stages.

Example:

class User < ApplicationRecord
  before_save :normalize_name

  def normalize_name
    self.name = name.downcase
  end
end

10. What is render vs. redirect_to in Rails?

Answer:

• render: Renders a view.
• redirect_to: Redirects to another URL.

Example:

render 'new'         # Renders new.html.erb
redirect_to users_path  # Redirects to index

11. What is the difference between .new and .create in Rails?

Answer:

• .new: Instantiates an object without saving.
• .create: Instantiates and saves in one step.

Example:

user = User.new(name: "John")  # Not saved
user.save

user = User.create(name: "John")  # Saved

12. How does Rails handle sessions and cookies?

Answer:

• Sessions store data across requests.
• Cookies store data in the user’s browser.

Example:

session[:user_id] = 1
cookies[:username] = "john"

13. What is CSRF protection in Rails?

Answer:
Rails includes Cross-Site Request Forgery (CSRF) protection using protect_from_forgery.

14. How to handle background jobs in Rails?

Answer:
Use Active Job with Sidekiq or Delayed Job.

Example:

class SendEmailJob < ApplicationJob
  queue_as :default

  def perform(user)
    UserMailer.welcome_email(user).deliver_now
  end
end

15. What is the purpose of config/routes.rb?

Answer:
It defines application routes.

Example:

Rails.application.routes.draw do
  resources :users
end

16. How to use concerns in Rails?

Answer:
Concerns allow reusing code across models or controllers.

Example:

module Taggable
  extend ActiveSupport::Concern

  included do
    has_many :tags
  end
end

class Post < ApplicationRecord
  include Taggable
end

17. What are scopes in Rails?

Answer:
Scopes define reusable queries.

Example:

class User < ApplicationRecord
  scope :active, -> { where(active: true) }
end

User.active  # Fetch active users

18. Explain has_many :through association.

Answer:
It connects models through a join table.

Example:

class Doctor < ApplicationRecord
  has_many :appointments
  has_many :patients, through: :appointments
end

class Appointment < ApplicationRecord
  belongs_to :doctor
  belongs_to :patient
end

19. What is Polymorphic Association in Rails?

Answer:
It allows a model to belong to multiple other models.

Example:

class Comment < ApplicationRecord
  belongs_to :commentable, polymorphic: true
end

class Post < ApplicationRecord
  has_many :comments, as: :commentable
end

class Video < ApplicationRecord
  has_many :comments, as: :commentable
end

20. What is the difference between pluck and select?

Answer:

• pluck: Fetches specific column values.
• select: Returns objects with attributes.

Example:

User.pluck(:name)  # Returns array of names
User.select(:name) # Returns ActiveRecord objects

21. What is dependent: :destroy in Rails?

Answer:
It ensures that associated records are deleted when the parent is deleted.

Example:

class User < ApplicationRecord
  has_many :posts, dependent: :destroy
end

When a User is deleted, all its posts will be deleted too.

22. What is before_action in Rails?

Answer:
It runs a method before specific controller actions.

Example:

class UsersController < ApplicationController
  before_action :authenticate_user, only: [:edit, :update]

  def authenticate_user
    redirect_to login_path unless logged_in?
  end
end

23. What is has_secure_password in Rails?

Answer:
It provides authentication features using bcrypt.

Example:

class User < ApplicationRecord
  has_secure_password
end

It automatically adds password hashing and authentication.

24. How does Rails handle time zones?

Answer:
Rails uses config.time_zone to manage time zones.

Example:

config.time_zone = "Eastern Time (US & Canada)"
Time.zone.now

25. What is flash in Rails?

Answer:
flash is used to show temporary messages between requests.

Example:

flash[:notice] = "User created successfully"
redirect_to users_path

26. What is the difference between symbol and string in Ruby?

Answer:

• Symbols (:name) are immutable and memory-efficient.
• Strings ("name") are mutable.

Example:

:name.object_id == :name.object_id # true
"hello".object_id == "hello".object_id # false

27. What are helpers in Rails?

Answer:
Helpers are methods used in views to keep logic out of templates.

Example:

module UsersHelper
  def full_name(user)
    "#{user.first_name} #{user.last_name}"
  end
end

28. What is a Concern in Rails?

Answer:
Concerns are modules that add shared behavior to models or controllers.

Example:

module Archivable
  extend ActiveSupport::Concern

  included do
    scope :archived, -> { where(archived: true) }
  end
end

29. What is enum in Rails?

Answer:
enum defines a set of possible values for an attribute.

Example:

class User < ApplicationRecord
  enum role: { admin: 0, user: 1, guest: 2 }
end

30. What is the difference between map, each, and collect in Ruby?

Answer:

• each: Iterates but does not modify the array.
• map / collect: Returns a new array with modified elements.

Example:

[1, 2, 3].map { |n| n * 2 } # [2, 4, 6]

31. What is the difference between find and where in Rails?

Answer:

• find: Fetches a single record by primary key.
• where: Returns an ActiveRecord relation (can be further chained).

Example:

User.find(1) # Returns a single user
User.where(name: "John") # Returns a relation (array-like)

32. What is the scope method in Rails?

Answer:
Scopes define reusable queries.

Example:

class User < ApplicationRecord
  scope :active, -> { where(active: true) }
end

33. What is joins in Rails?

Answer:
joins performs SQL joins between tables.

Example:

User.joins(:posts).where(posts: { published: true })

34. What is includes in Rails?

Answer:
includes preloads associations to avoid N+1 queries.

Example:

User.includes(:posts).each { |user| puts user.posts.count }

35. What is eager_load vs preload in Rails?

Answer:

• eager_load: Uses LEFT OUTER JOIN.
• preload: Runs separate queries.

Example:

User.eager_load(:posts) # Uses JOIN
User.preload(:posts)    # Uses separate queries

36. What is the difference between any?, present?, blank?, and empty? in Rails?

Answer:

• empty? → Checks if a collection is empty.
• blank? → Checks if a value is nil or empty.
• present? → Opposite of blank?.
• any? → Checks if an enumerable has at least one item.

37. What is a Rails API-only application?

Answer:
A Rails application without views, used for APIs.

Command:

rails new my_api --api

38. What is the difference between to_s, to_i, and to_sym?

Answer:

• to_s: Converts to string.
• to_i: Converts to integer.
• to_sym: Converts to symbol.

Example:

"10".to_i # 10
:hello.to_s # "hello"

39. How to set environment variables in Rails?

Answer:
Use the dotenv gem or Rails credentials.

Example:

ENV['API_KEY']

40. What is nested attributes in Rails?

Answer:
Allows saving related records via a parent model.

Example:

class User < ApplicationRecord
  has_one :profile
  accepts_nested_attributes_for :profile
end

41. What is the difference between update and update_attribute?

Answer:

• update: Updates multiple attributes and runs validations.
• update_attribute: Updates a single attribute but skips validations.

Example:

user.update(name: "John", email: "john@example.com")  # Runs validations
user.update_attribute(:name, "Mike")  # Skips validations

42. What is update_columns in Rails?

Answer:
update_columns updates multiple attributes without callbacks or validations.

Example:

user.update_columns(name: "Mike", email: "mike@example.com")

43. What is after_commit in Rails?

Answer:
after_commit runs a callback only after the database transaction is committed.

Example:

class User < ApplicationRecord
  after_commit :send_welcome_email, on: :create

  def send_welcome_email
    UserMailer.welcome_email(self).deliver_later
  end
end

44. What is pluck in Rails?

Answer:
pluck directly retrieves specific column values from the database as an array.

Example:

User.pluck(:name)  # => ["John", "Jane"]

45. What is default_scope in Rails?

Answer:
default_scope applies a default condition to all queries.

Example:

class User < ApplicationRecord
  default_scope { where(active: true) }
end

46. What is counter_cache in Rails?

Answer:
It stores the count of associated records in a separate column.

Example:

class Post < ApplicationRecord
  has_many :comments, counter_cache: true
end

This reduces database queries for counting.

47. What is the difference between has_many :through and has_and_belongs_to_many?

Answer:

• has_many :through: Uses a join model.
• has_and_belongs_to_many (HABTM): Direct many-to-many association without a join model.

Example:

class Doctor < ApplicationRecord
  has_many :appointments
  has_many :patients, through: :appointments
end

48. How does Rails handle CORS?

Answer:
Using the rack-cors gem to allow cross-origin requests.

Example (config/initializers/cors.rb):

Rails.application.config.middleware.insert_before 0, Rack::Cors do
  allow do
    origins '*'
    resource '*', headers: :any, methods: [:get, :post, :options]
  end
end

49. What is optimistic locking in Rails?

Answer:
It prevents concurrent updates from overriding each other.

Example:

class Product < ApplicationRecord
  attribute :lock_version, :integer
end

50. What is pessimistic locking in Rails?

Answer:
It locks a database row while being updated.

Example:

product = Product.lock.find(1)

51. What is delegate in Rails?

Answer:
delegate forwards method calls to another object.

Example:

class User < ApplicationRecord
  belongs_to :profile
  delegate :bio, to: :profile
end

Now user.bio will return profile.bio.

52. What is transaction in Rails?

Answer:
A transaction ensures multiple database operations are atomic.

Example:

ActiveRecord::Base.transaction do
  user.update!(balance: user.balance - 100)
  account.update!(balance: account.balance + 100)
end

53. What is touch in Rails?

Answer:
touch updates the updated_at timestamp of a record.

Example:

user.touch  # Updates updated_at

54. What is the difference between destroy_all and delete_all?

Answer:

• destroy_all: Runs callbacks and deletes records.
• delete_all: Directly deletes records from the database.

Example:

User.destroy_all # Triggers callbacks
User.delete_all  # Does not trigger callbacks

55. How to implement soft delete in Rails?

Answer:
Using the paranoia gem or adding a deleted_at column.

Example:

class User < ApplicationRecord
  def soft_delete
    update(deleted_at: Time.current)
  end
end

56. What is ActiveStorage in Rails?

Answer:
It handles file uploads natively in Rails.

Example:

class User < ApplicationRecord
  has_one_attached :avatar
end

57. What is the difference between present?, blank?, and nil??

Answer:

• nil? → True if the object is nil.
• blank? → True if nil or empty ("").
• present? → Opposite of blank?.

Example:

"".blank?   # true
"Hello".present? # true

58. How do you cache data in Rails?

Answer:
Using Rails.cache.

Example:

Rails.cache.fetch("user_#{user.id}") { user.name }

59. How to use enum with a default value in Rails?

Answer:
Use default: in migration.

Example:

class AddRoleToUsers < ActiveRecord::Migration[6.0]
  def change
    add_column :users, :role, :integer, default: 0
  end
end

60. What is yield in Ruby?

Answer:
yield passes control to a block.

Example:

def greet
  puts "Hello"
  yield
  puts "Goodbye"
end

greet { puts "How are you?" }

Output:

Hello
How are you?
Goodbye

61. What is Rails.application.config_for?

Answer:
It loads YAML configuration files from config/settings.yml.

Example:

settings = Rails.application.config_for(:database)
puts settings["host"]

62. What is turbo in Rails?

Answer:
Turbo (from Hotwire) speeds up page updates without JavaScript.

Example:

<%= turbo_frame_tag "posts" do %>
  <%= render @posts %>
<% end %>

63. What is stimulus.js in Rails?

Answer:
Stimulus.js is a lightweight JavaScript framework for enhancing Rails views.

Example:

import { Controller } from "@hotwired/stimulus"

export default class extends Controller {
  greet() {
    alert("Hello from Stimulus!")
  }
}

64. What is prepend_before_action?

Answer:
Runs a before_action before others in a controller.

Example:

class ApplicationController < ActionController::Base
  prepend_before_action :set_locale
end

65. What is Rails.cache.fetch?

Answer:
It caches expensive database queries.

Example:

Rails.cache.fetch("users_list", expires_in: 12.hours) do
  User.all.to_a
end

66. How do you use has_many :through with additional attributes?

Answer:
Use a join model with extra fields.

Example:

class Appointment < ApplicationRecord
  belongs_to :doctor
  belongs_to :patient
end

class Doctor < ApplicationRecord
  has_many :appointments
  has_many :patients, through: :appointments
end

67. How do you enable SQL logging in Rails?

Answer:
Use ActiveRecord::Base.logger.

Example:

ActiveRecord::Base.logger = Logger.new(STDOUT)

68. What is ActiveSupport::Concern?

Answer:
A cleaner way to include shared logic in models.

Example:

module Archivable
  extend ActiveSupport::Concern

  included do
    scope :archived, -> { where(archived: true) }
  end
end

69. What is ActiveRecord::Relation?

Answer:
It represents a database query that hasn’t been executed yet.

Example:

users = User.where(active: true) # This is an ActiveRecord::Relation
users.to_a # Executes the query

70. What is ActionCable in Rails?

Answer:
It enables WebSockets for real-time features like chat.

Example (channels/chat_channel.rb):

class ChatChannel < ApplicationCable::Channel
  def subscribed
    stream_from "chat_#{params[:room]}"
  end
end

71. How do you generate a Rails API-only app?

Answer:
Use the --api flag.

Command:

rails new my_api --api

72. What is rails credentials:edit?

Answer:
It securely stores API keys and secrets.

Command:

EDITOR=nano rails credentials:edit

73. How do you serve static assets in Rails?

Answer:
Enable config.public_file_server.enabled = true in production.rb.

74. What is bundle exec in Rails?

Answer:
It ensures that commands run using the correct Gemfile dependencies.

Example:

bundle exec rails server

75. What is shallow routing in Rails?

Answer:
It avoids nesting unnecessary routes.

Example:

resources :posts, shallow: true do
  resources :comments
end

76. What is dotenv-rails?

Answer:
It loads environment variables from .env files.

Example (.env file):

API_KEY=abcdef123456

77. How do you force SSL in Rails?

Answer:
Set force_ssl = true in config/environments/production.rb.

78. What is self.included(base) in Ruby modules?

Answer:
It allows executing code when a module is included in a class.

Example:

module Greeting
  def self.included(base)
    puts "#{base} included Greeting"
  end
end

79. How do you create a Rails task?

Answer:
Create a .rake file inside lib/tasks.

Example (lib/tasks/hello.rake):

namespace :greet do
  desc "Say hello"
  task hello: :environment do
    puts "Hello, World!"
  end
end

Run it with:

rake greet:hello

80. What is Rails.logger?

Answer:
It logs messages in log/development.log.

Example:

Rails.logger.info "This is a log message"

81. What is the difference between map, collect, and each in Ruby?

Answer:

• map and collect return a new array after applying the block to each element.
• each just iterates without returning a new array.

Example:

numbers = [1, 2, 3]

# map and collect
squares = numbers.map { |n| n * n }  # => [1, 4, 9]

# each
numbers.each { |n| puts n * n }  # Prints: 1 4 9

82. What is Rails.application.routes.draw?

Answer:
It defines the routes for a Rails application.

Example (config/routes.rb):

Rails.application.routes.draw do
  resources :users
end

83. How do you rescue exceptions globally in Rails?

Answer:
Use rescue_from in ApplicationController.

Example:

class ApplicationController < ActionController::Base
  rescue_from ActiveRecord::RecordNotFound, with: :not_found

  def not_found
    render json: { error: "Record not found" }, status: :not_found
  end
end

84. How to handle background jobs in Rails?

Answer:
Use ActiveJob with Sidekiq or DelayedJob.

Example:

class SendEmailJob < ApplicationJob
  queue_as :default

  def perform(user)
    UserMailer.welcome_email(user).deliver_now
  end
end

Enqueue job:

SendEmailJob.perform_later(user)

85. What is config.middleware.use in Rails?

Answer:
It adds custom middleware to the Rails request stack.

Example:

Rails.application.config.middleware.use MyCustomMiddleware

86. What is after_initialize in Rails?

Answer:
It runs code after Rails is fully initialized.

Example (config/initializers/custom.rb):

Rails.application.config.after_initialize do
  puts "Rails has started!"
end

87. How do you debug Rails applications?

Answer:
Use byebug or pry.

Example:

def show
  byebug
  @user = User.find(params[:id])
end

Start the server and trigger the action to pause execution.

88. What is rake db:migrate:status?

Answer:
It shows the status of all migrations.

Command:

rake db:migrate:status

89. How do you rollback a migration in Rails?

Answer:
Use rails db:rollback.

Command:

rails db:rollback STEP=1

90. What is the difference between rake and rails commands?

Answer:

• rake: Runs tasks without needing Rails booted.
• rails: Runs tasks that require Rails booted.

Example:

rake db:migrate  # Can also be done using:
rails db:migrate

91. What is rails notes?

Answer:
It finds TODO, FIXME, and OPTIMIZE comments in your code.

Command:

rails notes

92. What is Rails.application.eager_load!?

Answer:
It loads all application classes eagerly.

Example:

Rails.application.eager_load!

93. How do you use include, extend, and prepend in Ruby modules?

Answer:

• include: Adds methods as instance methods.
• extend: Adds methods as class methods.
• prepend: Overrides methods from the beginning of the lookup chain.

Example:

module Greetings
  def greet
    "Hello!"
  end
end

class User
  include Greetings  # Adds instance method
end

class Admin
  extend Greetings  # Adds class method
end

user = User.new
puts user.greet  # "Hello!"

puts Admin.greet  # "Hello!"

94. What is Rails.application.config.active_record.belongs_to_required_by_default?

Answer:
It forces belongs_to associations to require presence by default.

Example (config/application.rb):

Rails.application.config.active_record.belongs_to_required_by_default = false

95. What is foreign_key in Rails migrations?

Answer:
It enforces referential integrity at the database level.

Example:

add_foreign_key :orders, :users

96. What is up, down, and change in Rails migrations?

Answer:

• up: Runs migration forward.
• down: Rolls it back.
• change: Runs forward and reverses automatically.

Example:

class AddAgeToUsers < ActiveRecord::Migration[6.0]
  def change
    add_column :users, :age, :integer
  end
end

97. What is render json: in Rails?

Answer:
It returns JSON responses in controllers.

Example:

def show
  render json: { user: @user }
end

98. What is where.not in ActiveRecord?

Answer:
It filters records excluding certain conditions.

Example:

User.where.not(role: "admin")

99. How do you preload associations in Rails?

Answer:
Use includes, joins, or eager_load.

Example:

User.includes(:posts).all

100. What is rails g migration?

Answer:
It generates a migration file.

Command:

rails g migration AddAgeToUsers age:integer

Preparing for a Ruby on Rails interview requires a deep understanding of the framework’s core features, best practices, and real-world applications. This guide has covered 100 essential interview questions, ranging from basic concepts to advanced topics like ActiveRecord associations, caching strategies, background jobs, and API optimizations.

To ace your Rails interview:
✅ Practice coding with hands-on projects.
✅ Understand Rails internals, especially how ActiveRecord, routing, and controllers work.
✅ Stay updated with the latest Rails versions and best practices.

Bookmark this guide and keep revisiting these questions to strengthen your knowledge. Good luck with your interview! 🚀💡