Top 100 Ruby on Rails Interview Question and Answers

1. What is Ruby on Rails?

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

2. What are the advantages of using Ruby on Rails?

Answer:

• Follows MVC architecture for modular code.
• DRY and CoC principles improve code efficiency.
• Built-in tools for database migration, scaffolding, and testing.
• Large community and extensive libraries (gems).

3. Explain the MVC architecture in Rails.

Answer:

• Model: Handles data and business logic.
• View: Manages the presentation layer (HTML, ERB templates).
• Controller: Manages the application logic, processes user input, and interacts with the model and view.

4. What is Active Record?

Answer: Active Record is the ORM (Object-Relational Mapping) layer in Rails, which facilitates database interaction by representing data as objects.

5. What is a Rails gem?

Answer: Gems are libraries or plugins that extend the functionality of a Ruby or Rails application.

6. What is the difference between render and redirect_to in Rails?

Answer:

• render: Renders a specific view template without making a new HTTP request.
• redirect_to: Sends an HTTP redirect to the browser, which initiates a new request to a different URL.

7. What are Rails migrations?

Answer: Rails migrations are used to make changes to the database schema over time in a consistent and version-controlled manner.

8. How does the has_many :through association work?

Answer: It sets up a many-to-many connection between models using a third model to manage the relationship.

9. What is the Rails Asset Pipeline?

Answer: The asset pipeline is used to manage and concatenate assets such as JavaScript, CSS, and images in a Rails application.

10. What are Rails callbacks?

Answer: Callbacks are methods that get triggered at specific stages of an object’s lifecycle (e.g., before_save, after_create).

11. Explain eager loading and lazy loading in Rails.

Answer:

• Eager loading: Loads all associated records in a single query (e.g., includes).
• Lazy loading: Loads associated records when they are accessed (e.g., where or find).

12. How does Rails handle security vulnerabilities?

Answer: Rails includes protections against common vulnerabilities like SQL injection, XSS (Cross-Site Scripting), and CSRF (Cross-Site Request Forgery) by default.

13. What are polymorphic associations in Rails?

Answer: Polymorphic associations allow a model to belong to multiple other models using a single association.

14. What is the difference between save and save!?

Answer:

• save: Returns true or false based on validation.
• save!: Raises an exception if validation fails.

15. What is Turbolinks in Rails?

Answer: Turbolinks speeds up navigation by loading only the body and merging it into the current page without reloading JavaScript or CSS.

16. How do you set up a custom validation in Rails?

Answer: Define a method in the model and use validate.

ruby

`class User < ApplicationRecord

validate :email_domain_check

private
def email_domain_check

1
errors.add(:email, "must be from example.com") unless email.include?("example.com")  

end

end`

17. How do you handle file uploads in Rails?

Answer: Use gems like CarrierWave or ActiveStorage.

18. How do you optimize database queries in Rails?

Answer:

• Use includes for eager loading.
• Use select to fetch only necessary columns.
• Avoid N+1 queries.

19. What is a Rails Concern? How do you use it?

Answer: Concerns are modules used to extract reusable code in models and controllers. They are included using include or extend.

20. How do you implement caching in Rails?

Answer:

• Fragment caching: Cache parts of a view.
• Action caching: Cache the entire output of an action.
• Use libraries like Memcached or Redis.

21. Describe a challenging bug you resolved in Rails.

Answer: Be prepared to talk about debugging techniques like using byebug, reviewing logs, or isolating issues in test environments.

22. How do you ensure scalability in Rails applications?

Answer: Use caching, optimize database queries, and implement background jobs using tools like Sidekiq.

23. Explain the Rails initialization process.

Answer: It involves loading the application.rb file, initializing gems, and loading middleware, routes, and models.

24. How do you handle background jobs in Rails?

Answer: Use tools like Sidekiq, Delayed Job, or Resque for asynchronous processing.

25. How do you implement RESTful APIs in Rails?

Answer: Use Rails controllers, ActiveModel::Serializer, and tools like JBuilder for JSON responses.

26. What are ActionCable and WebSockets in Rails?

Answer:

• ActionCable: A Rails framework for real-time communication over WebSockets.
• WebSockets: A protocol for two-way communication between the client and server without repeatedly making HTTP requests.
• ActionCable integrates WebSockets with the Rails app and allows broadcasting messages to connected clients.

27. How do you integrate a third-party API in Rails?

Answer: Use the Net::HTTP, Faraday, or HTTParty gem to make API requests. For example:

ruby

`require ‘httparty’

response = HTTParty.get(‘https://api.example.com/resource’)

puts response.body`

28. What are some best practices for writing Rails tests?

Answer:

• Use RSpec or Minitest for testing.
• Follow TDD principles (Test-Driven Development).
• Mock and stub external APIs.
• Use FactoryBot for test data.
• Organize tests into unit, integration, and system levels.

29. How do you implement authentication and authorization in Rails?

Answer:

• Authentication: Use Devise or OmniAuth for login systems.
• Authorization: Use Pundit or CanCanCan for role-based access control.

30. What is the difference between form_for and form_with?

Answer:

• form_for: Used for creating forms associated with a specific model.
• form_with: Introduced in Rails 5.1, supports forms with or without models and uses modern syntax.

31. What is the role of the config/routes.rb file?

Answer: It defines URL endpoints and maps them to controller actions. Example:

ruby

`Rails.application.routes.draw do

resources :articles

end`

32. How do you debug a Rails application?

Answer:

• Use byebug or pry to inspect the code.
• Check logs in the log/ directory.
• Use tools like better_errors or rack-mini-profiler.

33. What is a migration rollback? How do you use it?

Answer: A rollback undoes the last migration. Use the command:

bash

rails db:rollback

34. How do you optimize asset loading in Rails?

Answer:

• Compress assets using tools like Uglifier or CSS compressors.
• Use the Rails asset pipeline for concatenation and minification.
• Leverage browser caching with proper cache headers.

35. What is a Rails initializer?

Answer: Initializers are Ruby files in the config/initializers/ directory used to set up application-wide configurations during boot.

36. How do you handle time zones in Rails?

Answer:

• Set the application time zone in config/application.rb:

  ruby

  config.time_zone = 'UTC'

• Use Time.zone.now for timezone-aware operations.

37. How do you manage background jobs in Rails?

Answer: Use gems like Sidekiq or Delayed Job. Define jobs and enqueue them for execution. Example:

ruby

`class MyJob < ApplicationJob

def perform

1
# Task logic here 

end

end`

38. What are Rails Environments?

Answer: Rails has three default environments:

• Development: Used for coding and debugging.
• Test: Used for running automated tests.
• Production: Used for the live application.

39. What is the params object in Rails?

Answer: The params object contains parameters sent by the client in HTTP requests (e.g., form data, query strings).

40. What is the purpose of Rails layouts?

Answer: Layouts provide a common structure (e.g., header, footer) for views in an application.

41. What is Spring in Rails?

Answer: Spring is a Rails application preloader that speeds up development by keeping the app running in the background.

42. How do you secure sensitive data in a Rails application?

Answer:

• Use environment variables for secrets (e.g., dotenv gem).
• Store credentials using rails credentials:edit.
• Use strong parameter filtering in controllers.

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

Answer:

• delete: Removes a record directly from the database without callbacks.
• destroy: Removes a record and runs associated callbacks.

44. How do you handle file attachments in Rails?

Answer: Use Active Storage to manage file uploads and attachments. Example:

ruby

has_one_attached :image

45. How do you implement pagination in Rails?

Answer: Use gems like kaminari or will_paginate. Example with Kaminari:

ruby

@posts = Post.page(params[:page]).per(10)

46. What is before_action in Rails?

Answer: A callback that runs a specified method before controller actions.

ruby

before_action :authenticate_user

47. What are helpers in Rails?

Answer: Helpers are methods used to simplify view templates.

48. What are partials in Rails?

Answer: Partials are reusable view templates prefixed with an underscore (e.g., _form.html.erb).

49. What is a nested resource in Rails?

Answer: A resource that belongs to another resource. Example:

ruby

`resources :articles do

resources :comments

end`

50. How do you validate uniqueness in Rails models?

Answer: Use the validates method. Example:

ruby

validates :email, uniqueness: true

51. What are polymorphic routes in Rails?

Answer: Routes that handle multiple controllers based on a single resource.

52. What are Rails engines?

Answer: Mini Rails applications that can be mounted in a parent app.

53. What is a scope in Rails?

Answer: A custom query defined in a model. Example:

ruby

scope :active, -> { where(active: true) }

54. What is ActiveSupport?

Answer: A Ruby library providing utility classes and extensions used by Rails.

55. What is an N+1 query problem in Rails?

Answer:
The N+1 query problem occurs when the application executes one query to retrieve the main records and then N additional queries to retrieve related records, causing inefficiency.

Solution: Use includes to eager load associations:

ruby

@posts = Post.includes(:comments)

56. How do you perform eager loading in Rails?

Answer: Use the includes or joins method in ActiveRecord to load associated records in fewer queries.

Example:

ruby

@users = User.includes(:posts).all

57. What is the purpose of the rake command in Rails?

Answer:
The rake command is used to run administrative tasks like database migrations, cleaning logs, or running custom tasks.

Example:

bash

rake db:migrate

58. What are joins and includes in Rails? How are they different?

Answer:

• joins: Performs an SQL join but doesn’t load associated records.
• includes: Eager loads associated records to prevent N+1 queries.
• Example:

ruby

`# joins

User.joins(:posts)

#includes

User.includes(:posts)`

59. How do you define a custom route in Rails?

Answer: Use the get, post, or other HTTP verbs in config/routes.rb.
Example:

ruby

get 'custom_route', to: 'controller#action'

60. How do you handle exceptions in Rails?

Answer: Use rescue_from in controllers to handle specific exceptions.

Example:

ruby

`class ApplicationController < ActionController::Base

rescue_from ActiveRecord::RecordNotFound, with: :record_not_found

def record_not_found

1
render plain: '404 Not Found', status: 404

end

end`

61. What is Rails’ asset pipeline?

Answer:
The asset pipeline concatenates, minifies, and serves CSS, JavaScript, and images efficiently. Files are stored in the app/assets, lib/assets, or vendor/assets directories.

62. How do you create a custom validation in Rails?

Answer: Define a custom validation method in the model.

Example:

ruby

`class User < ApplicationRecord

validate :email_must_be_valid

def email_must_be_valid

1
errors.add(:email, 'is not valid') unless email.include?('@')

end

end`

63. What is the difference between save and save! in Rails?

Answer:

• save: Returns false if validation fails.
• save!: Raises an exception if validation fails.

64. What are filters in Rails controllers?

Answer: Filters are methods used to run code before or after controller actions.
Examples:

• before_action
• after_action
• around_action

65. What is a Concern in Rails?

Answer: A module that provides shared functionality across models or controllers.
Example:

ruby

`module Archivable

extend ActiveSupport::Concern

included do

1
scope :archived, -> { where(archived: true) }

end

end`

66. How do you deploy a Rails application?

Answer:

• Use tools like Capistrano, Docker, or Heroku.
• Precompile assets using rails assets:precompile.
• Run database migrations with rails db:migrate.

67. What is caching in Rails, and how do you implement it?

Answer:
Caching stores frequently accessed data to improve performance. Rails supports:

• Fragment Caching
• Page Caching
• Action Caching

Example for fragment caching:

erb

`<% cache @article do %>

<%= render @article %>

<% end %>`

68. How do you generate a migration in Rails?

Answer: Use the rails generate migration command.

Example:

bash

rails generate migration AddAgeToUsers age:integer

69. What is a database schema in Rails?

Answer:
The schema.rb file represents the current state of the database structure, including tables, columns, and indexes.

70. How do you use ActiveRecord callbacks?

Answer: Callbacks are methods triggered during the lifecycle of an ActiveRecord object.

Examples:

• before_save
• after_create
• around_destroy

71. What is the difference between find and find_by?

Answer:

• find: Finds a record by its primary key. Raises an error if not found.
• find_by: Finds the first record matching the condition. Returns nil if not found.

72. How do you implement nested forms in Rails?

Answer: Use fields_for and enable accepts_nested_attributes_for in the model.
Example:

ruby

`class Post < ApplicationRecord

has_many :comments

accepts_nested_attributes_for :comments

end`

73. What is the difference between delete and destroy_all?

Answer:

• delete: Removes records without callbacks.
• destroy_all: Deletes records and runs callbacks.

74. How do you generate JSON responses in Rails?

Answer: Use render json.
Example:

ruby

render json: @user

75. What is CSRF, and how does Rails prevent it?

Answer:
Cross-Site Request Forgery (CSRF) is an attack where unauthorized commands are transmitted from a user. Rails includes CSRF protection using authenticity tokens.

76. How do you handle file uploads in Rails?

Answer: Use Active Storage or gems like CarrierWave.

77. What is the purpose of flash in Rails?

Answer:
flash stores temporary data for use in the next request, often for displaying messages.

78. How do you use Rails validations?

Answer: Add validations in the model.
Example:

ruby

validates :name, presence: true, uniqueness: true

79. How do you schedule tasks in Rails?

Answer: Use gems like whenever or sidekiq-scheduler.

80. How do you perform database indexing in Rails?

Answer: Add indexes via migrations:

bash

rails generate migration AddIndexToUsersEmail

81. What is the has_secure_password method in Rails?

Answer:
The has_secure_password method adds methods to set and authenticate securely hashed passwords using bcrypt. It requires a password_digest column in the database.
Example:

ruby

`class User < ApplicationRecord

has_secure_password

end`

82. What is the purpose of ActiveJob in Rails?

Answer:
ActiveJob is a framework for declaring background jobs and making them run on various queuing backends (e.g., Sidekiq, DelayedJob).
Example:

ruby

`class MyJob < ApplicationJob

queue_as :default

def perform(*args)

1
# Do something

end

end`

83. How do you configure internationalization (i18n) in Rails?

Answer:
Rails has built-in support for internationalization. Create locale files in config/locales/.
Example:

yml

`# config/locales/en.yml

en:

hello: “Hello World”`

Usage in views:

erb

<%= t('hello') %>

84. What are concerns in Rails, and when do you use them?

Answer:
Concerns are modules used to extract and reuse common functionality across models or controllers. They help keep code DRY (Don’t Repeat Yourself).
Example:

ruby

`module Archivable

extend ActiveSupport::Concern

included do

1
scope :archived, -> { where(archived: true) }

end

end`

85. What is the difference between optimistic and pessimistic locking in Rails?

Answer:

• Optimistic locking: Uses a lock_version column to ensure no conflicting updates.
• Pessimistic locking: Locks a database row until the transaction is complete, preventing others from modifying it.
  Example of pessimistic locking:

ruby

user = User.lock("FOR UPDATE").find(1)

86. What is the difference between polymorphic and single table inheritance (STI) in Rails?

Answer:

• Polymorphic associations: Allow a model to belong to multiple other models using a shared interface.
• STI: Allows multiple classes to inherit from a single database table.

87. What is a PORO in Rails?

Answer:
PORO stands for “Plain Old Ruby Object.” It refers to simple Ruby classes that aren’t dependent on Rails. They are used for business logic that doesn’t fit into models or controllers.

88. How do you use Rails helpers?

Answer:
Helpers are used to encapsulate reusable view logic.
Example:

ruby

`module ApplicationHelper

def format_date(date)

1
date.strftime('%d-%m-%Y')

end

end`

Usage in views:

erb

<%= format_date(Time.now) %>

89. What is the difference between before_validation and before_save callbacks?

Answer:

• before_validation: Runs before validations are checked.
• before_save: Runs before the record is saved to the database.

90. What are partials in Rails?

Answer:
Partials are reusable view templates.
Example: Create _form.html.erb and render it using:

erb

<%= render 'form' %>

91. How do you debug a Rails application?

Answer:
Use tools like:

• byebug or debugger gem for setting breakpoints.
• Logs (log/development.log).
• Browser developer tools for frontend issues.

92. What is the difference between render and redirect_to?

Answer:

• render: Renders a specific template without making a new HTTP request.
• redirect_to: Issues a new HTTP request to a different action or URL.

93. What are scopes in Rails?

Answer:
Scopes are methods for defining reusable queries in models.

Example:

ruby

scope :active, -> { where(active: true) }

94. What is Rails’ ActiveSupport::Concern?

Answer:
It is a module used to include shared functionality in classes. It provides included and class_methods blocks for extending functionality.

95. How do you create a custom generator in Rails?

Answer:
Create a generator under the lib/generators directory.
Example:

bash

rails generate generator MyGenerator

96. What is the purpose of environment.rb in Rails?

Answer:
The environment.rb file initializes the Rails application and loads all configurations.

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

Answer:

• has_many :through: Requires an intermediate join model.
• has_and_belongs_to_many: Direct many-to-many relationship without an intermediate model.

98. How do you handle background jobs in Rails?

Answer:
Use gems like Sidekiq, Resque, or DelayedJob to manage background jobs.

99. What is a serializer in Rails?

Answer:
Serializers format data for APIs. Rails supports ActiveModel::Serializer.

Example:

ruby

1
`class UserSerializer <ActiveModel::Serializer   attributes :id, :name, :email end` 

100. What are Action Mailers in Rails?

Answer:
Action Mailers are used to send and receive emails in Rails.

Example:

ruby

1
`class UserMailer < ApplicationMailer

def welcome_email(user)

1
2
3
4
@user = user

mail(to: @user.email, subject: 'Welcome to My 
App')   end

end`