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Ruby on Rails Interview Questions

Hiring + recruiting | Blog Post

15 Ruby on Rails Interview Questions for Hiring Ruby on Rails Engineers

Todd Adams

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Hiring the right Ruby on Rails engineer requires carefully crafted interview questions that assess both fundamental and advanced knowledge of the framework. This list of Ruby on Rails interview questions is designed to help identify candidates with the necessary skills and experience to excel in a Ruby on Rails role, ensuring they can build, maintain, and scale web applications effectively.

Ruby on Rails Interview Questions

1. What is Ruby on Rails and how does it follow the MVC architecture?

Question Explanation: Understanding Ruby on Rails and its architectural pattern is fundamental for any Rails developer. This question probes the candidate’s basic knowledge of the framework and its core design philosophy.

Expected Answer: Ruby on Rails, often just called Rails, is a server-side web application framework written in Ruby. It is designed to make programming web applications easier by making assumptions about what every developer needs to get started. Rails follows the Model-View-Controller (MVC) architectural pattern, which separates the application into three interconnected components.

  • Model: Represents the data and the logic to manipulate that data. In Rails, this is handled by ActiveRecord, which provides an interface and binding between the tables in a relational database and the Ruby program code that manipulates database records.
  • View: Represents the presentation layer, responsible for displaying the data to the user. In Rails, this is typically handled by Embedded Ruby (ERB) templates that generate HTML.
  • Controller: Acts as an intermediary between Model and View. It processes incoming requests, manipulates data using the Model, and renders the appropriate View. In Rails, controllers are Ruby classes that extend the ApplicationController and define actions that correspond to user requests.

Evaluating Responses:

  • The candidate should clearly explain Ruby on Rails as a web application framework and its main purpose.
  • Look for a precise explanation of the MVC architecture and how it applies to Rails.
  • A good answer to Ruby on Rails Interview questions should include specific details about how Models, Views, and Controllers interact in a Rails application.

2. Can you explain the concept of “Convention over Configuration” in Ruby on Rails?

Question Explanation:

This question tests the candidate’s understanding of one of the core principles of Rails, which aims to reduce the number of decisions developers need to make.

Expected Answer:

“Convention over Configuration” (CoC) is a software design paradigm used by Rails to reduce the complexity of software development by favoring convention and default behavior over explicit configuration. Rails provides a standard set of conventions that, if followed, allow developers to work without having to specify every single detail. For example:

  • Default file structures: Models are placed in the app/models directory, views in app/views, and controllers in app/controllers.
  • Naming conventions: Database tables are named in plural (e.g., users for a User model), and primary keys are named id.
  • Automated tasks: Rails can automatically link models to database tables, controllers to views, and routes to controller actions without additional configuration if naming conventions are followed.

Evaluating Responses:

  • The candidate should clearly articulate the CoC principle and its role in simplifying development.
  • Look for examples of conventions in Rails and how they minimize the need for configuration.
  • A good answer should convey an understanding of how CoC increases productivity and reduces the likelihood of errors.

3. How does ActiveRecord work in Ruby on Rails?

Question Explanation:

ActiveRecord is a key component of Rails, and this Ruby on Rails interview question assesses the candidate’s understanding of how it functions as an ORM (Object-Relational Mapping) tool.

Expected Answer:

ActiveRecord is the ORM layer supplied with Rails, responsible for representing business data and logic. It provides a powerful API to interact with database records as Ruby objects, following the Active Record design pattern. Key features include:

  • Table Mapping: Each model class in Rails corresponds to a database table.
  • Object Instantiation: Each instance of a model corresponds to a row in the database table.
  • CRUD Operations: ActiveRecord provides methods to create, read, update, and delete records without writing SQL queries. For example, User.create(name: "Alice") creates a new user, User.find(1) retrieves a user by ID, and user.update(name: "Bob") updates the user’s name.
  • Associations: ActiveRecord supports associations between models, such as belongs_to, has_many, and has_one, facilitating complex queries and data manipulations.

Evaluating Responses:

  • The candidate should explain ActiveRecord’s role as an ORM and its primary functions.
  • Look for an understanding of how ActiveRecord maps tables to classes and rows to objects.
  • A comprehensive answer should include examples of CRUD operations and associations.

4. What are migrations in Ruby on Rails and why are they important?

Question Explanation:

Migrations are essential for managing database schema changes in Rails. This question evaluates the candidate’s understanding of how migrations work and their significance.

Expected Answer:

Migrations in Ruby on Rails are a convenient way to alter the database schema over time in a consistent and easy manner. They are Ruby classes that allow you to define changes to the database schema using a DSL (Domain-Specific Language), which can then be applied using Rake tasks. Migrations are important because:

  • Version Control: They allow you to version control your database schema, making it easier to track changes and collaborate with other developers.
  • Schema Management: You can create, alter, and drop database tables and columns without writing raw SQL. For example, rails generate migration AddEmailToUsers email:string generates a migration to add an email column to the users table.
  • Rollbacks: Migrations support rollbacks, enabling you to undo changes if something goes wrong. This is crucial for maintaining database integrity and consistency.

Evaluating Responses:

  • The candidate should explain what migrations are and how they are used in Rails.
  • Look for a clear description of the benefits of using migrations, including version control and ease of schema management.
  • A strong answer should mention the ability to roll back migrations and provide an example of a typical migration task.

5. How would you implement authentication in a Ruby on Rails application?

Question Explanation:

Authentication is a fundamental part of most web applications. This Ruby on Rails Interview question assesses the candidate’s knowledge of common methods and tools for implementing authentication in Rails.

Expected Answer:

To implement authentication in a Ruby on Rails application, you can use various methods and libraries. One of the most popular options is the Devise gem. Here’s a step-by-step outline of how you would typically use Devise for authentication:

  1. Add Devise to your Gemfile:
gem 'devise'

Then run bundle install.

  1. Install Devise:

Run the generator to set up Devise:

rails generate devise:install
  1. Configure Devise:

Follow the instructions generated by Devise, which might include setting up your mailer configuration and ensuring your root route is defined.

  1. Generate User model:

To create a User model with Devise, run:

rails generate devise User
  1. Run Migrations:

Apply the changes to the database:

rails db:migrate
  1. Add Devise Views:

Optionally, you can generate the views to customize the user interface:

rails generate devise:views
  1. Configure Routes:

Ensure your routes include Devise for the User model in config/routes.rb:

devise_for :users

After these steps, you have basic authentication set up, where users can register, log in, and log out.

Evaluating Responses:

  • The candidate should mention the use of the Devise gem, which is a widely adopted solution for authentication in Rails.
  • Look for a clear understanding of the steps involved in setting up Devise.
  • A good answer should also mention customizing Devise views and configurations.

6. Can you explain the difference between render and redirect_to in Rails controllers?

Question Explanation:

Understanding the difference between render and redirect_to is crucial for correctly managing HTTP responses and flow control in a Rails application.

Expected Answer:

In Rails, render and redirect_to are used to control the response to a client’s request, but they serve different purposes:

  • render: This method is used to render a template without initiating a new HTTP request. It can render views, partials, and other formats directly. For example:
render :new

This will render the new.html.erb template within the current request-response cycle. It does not cause the browser to issue a new request, so the URL in the browser remains the same.

  • redirect_to: This method is used to redirect the browser to a different action, controller, or URL. It initiates a new HTTP request, causing the browser to change the URL. For example:
redirect_to action: :index

This sends a 302 HTTP status code by default and tells the browser to make a new request to the specified URL.

Evaluating Responses:

  • The candidate should clearly differentiate between rendering a view and redirecting to a different URL.
  • Look for examples demonstrating render and redirect_to usage.
  • A comprehensive answer should mention the impact on the HTTP request-response cycle and URL changes.

7. What is the asset pipeline in Ruby on Rails and how does it help in managing assets?

Question Explanation:

The asset pipeline is a key feature in Rails for managing CSS, JavaScript, and image assets. This Ruby on Rails interview question tests the candidate’s understanding of its purpose and benefits.

Expected Answer:

The asset pipeline in Ruby on Rails is a framework for managing and serving web assets, such as CSS, JavaScript, and images. It offers several advantages:

  • Concatenation and Minification: The asset pipeline combines multiple files into one and minifies them to reduce the number of HTTP requests and decrease load times. For example, it can concatenate all JavaScript files into a single file and minify it.
  • Preprocessing: It allows the use of preprocessing languages such as Sass for CSS and CoffeeScript for JavaScript. These preprocessors extend the capabilities of CSS and JavaScript and then compile into regular CSS and JS.
  • Fingerprints for Caching: Rails adds a unique fingerprint to each asset’s filename based on its content. This helps in caching because when the content changes, the fingerprint changes, ensuring the browser fetches the latest version.

To use the asset pipeline, assets should be placed in the app/assets, lib/assets, or vendor/assets directories. They are then included in views using helpers like javascript_include_tag and stylesheet_link_tag.

Evaluating Responses:

  • The candidate should explain the role of the asset pipeline in managing web assets.
  • Look for specific benefits such as concatenation, minification, preprocessing, and fingerprinting.
  • A good answer should include how the asset pipeline improves performance and maintainability.

8. How do you handle background jobs in Ruby on Rails?

Question Explanation: Handling background jobs efficiently is crucial for performance and scalability. This Ruby on Rails interview question assesses the candidate’s knowledge of background job processing in Rails.

Expected Answer: In Ruby on Rails, background jobs can be managed using various job processing libraries, with Sidekiq being one of the most popular choices. Here’s how you would typically handle background jobs using Sidekiq:

  1. Add Sidekiq to your Gemfile:
gem 'sidekiq'

Then run bundle install.

  1. Configure Sidekiq:

Create a configuration file (e.g., config/sidekiq.yml) and set up the necessary options. For example:

:concurrency: 5
:queues:
  - default
  1. Set up Redis:

Sidekiq requires Redis for job storage. Ensure Redis is installed and running on your system

  1. Create a Job:

Generate a new job using the Rails generator:

rails generate job ExampleJob

Implement the job logic in the generated file (app/jobs/example_job.rb):

class ExampleJob < ApplicationJob
  queue_as :default

  def perform(*args)
    # Job logic here
  end
end
  1. Enqueue the Job:

Enqueue the job from your controllers or models:

ExampleJob.perform_later(arg1, arg2)
  1. Start Sidekiq:

Run Sidekiq to start processing jobs:

bundle exec sidekiq

Evaluating Responses:

  • The candidate should mention Sidekiq or a similar job processing library.
  • Look for a clear outline of the steps involved in setting up and using Sidekiq.
  • A good answer should include details on configuring Redis, creating and enqueuing jobs, and running Sidekiq.

9. What are concerns in Ruby on Rails and how are they used?

Question Explanation: Concerns are a way to modularize and organize code in Rails. This question tests the candidate’s understanding of how to use concerns to keep the codebase clean and maintainable.

Expected Answer: Concerns in Ruby on Rails are modules that allow you to extract and encapsulate reusable code from models or controllers. They help in adhering to the Single Responsibility Principle and keep classes small and focused. Concerns are typically used when you want to share behavior among multiple models or controllers.

Steps to create and use concerns:

  1. Define a Concern:

Concerns are placed in the app/models/concerns or app/controllers/concerns directory. Create a module that defines the shared behavior. For example:

# app/models/concerns/trackable.rb
module Trackable
  extend ActiveSupport::Concern

  included do
    has_many :tracks
  end

  def track_event(event)
    tracks.create(event: event)
  end
end
  1. Include the Concern:

Include the concern in the models or controllers where you want to reuse the behavior.

# app/models/user.rb
class User < ApplicationRecord
  include Trackable
end

# app/models/post.rb
class Post < ApplicationRecord
  include Trackable
end
  1. Use the Concern Methods:

Now you can use the methods defined in the concern in your models.

user = User.find(1)
user.track_event('login')

Evaluating Responses:

  • The candidate should explain what concerns are and their purpose in Rails.
  • Look for a clear example of defining and including a concern.
  • A good answer should emphasize the benefits of using concerns for code reuse and maintainability.

10. How does Rails handle session management and what are the options for storing session data?

Question Explanation:

Session management is crucial for maintaining state in web applications. This question assesses the candidate’s understanding of how Rails handles sessions and the various storage options.

Expected Answer:

Rails handles session management using cookies by default, but it supports various options for storing session data. The session data can be stored in several ways, including:

  1. CookieStore (Default):

Stores session data on the client-side in a secure, encrypted cookie. It is suitable for small amounts of data.

Rails.application.config.session_store :cookie_store, key: '_your_app_session'
  1. ActiveRecordStore:

Stores session data in the database, useful for larger data or when you need to share session data across multiple servers.

Rails.application.config.session_store :active_record_store, key: '_your_app_session'
  1. CacheStore:

Stores session data in the Rails cache, which can be configured to use memory, Memcached, or Redis.

Rails.application.config.session_store :cache_store, key: '_your_app_session'
  1. RedisStore:

Uses Redis to store session data, providing high performance and scalability.

Rails.application.config.session_store :redis_store, servers: "redis://localhost:6379/0/session"

Evaluating Responses:

  • The candidate should explain how Rails handles session management and mention the default CookieStore.
  • Look for knowledge of different session storage options and their use cases.
  • A good answer should provide examples of configuring different session stores in a Rails application.

11. What is the purpose of the Gemfile and Bundler in Ruby on Rails?

Question Explanation: The Gemfile and Bundler are essential for managing dependencies in Rails applications. This question assesses the candidate’s understanding of these tools and their importance.

Expected Answer: The Gemfile and Bundler are used to manage dependencies in a Ruby on Rails application.

  1. Gemfile:

The Gemfile is a file that specifies the gems (libraries) required for your application to run. It includes details about the gem name and version. For example:

source 'https://rubygems.org'

gem 'rails', '~> 6.0.0'
gem 'pg', '>= 0.18', '< 2.0'
gem 'puma', '~> 4.1'
gem 'sass-rails', '>= 6'
gem 'webpacker', '~> 4.0'
  1. Bundler:

Bundler is a dependency management tool that ensures all the required gems are installed in the correct versions specified in the Gemfile. It creates a Gemfile.lock file that records the exact versions of the gems installed, ensuring consistency across different environments.

Using Bundler:

  • To install the gems specified in the Gemfile, run:
bundle install
  • To update gems to their latest versions within the specified constraints:
bundle update
  • To add a new gem to the Gemfile and install it:
bundle add gem_name

Evaluating Responses:

  • The candidate should explain the role of the Gemfile in listing application dependencies.
  • Look for an understanding of how Bundler manages these dependencies and ensures consistency.
  • A good answer should include examples of common Bundler commands and their purposes.

12. Can you explain how routing works in Ruby on Rails?

Question Explanation:

Routing is a fundamental aspect of Rails, directing incoming requests to the appropriate controller actions. This question evaluates the candidate’s knowledge of Rails routing and how it is configured.

Expected Answer:

In Ruby on Rails, routing is handled by the config/routes.rb file, which maps incoming HTTP requests to controller actions. Rails uses a Domain-Specific Language (DSL) to define routes.

Basic Routing:

  1. Route Definition:

Routes are defined using the get, post, put, patch, delete, and resources methods. For example:

get 'welcome/index'
post 'users/create'
  1. Resourceful Routing:

Rails provides a way to define standard CRUD routes for a resource using the resources method:

resources :articles

This generates the following routes:

  • GET /articles (index)
  • GET /articles/new (new)
  • POST /articles (create)
  • GET /articles/:id (show)
  • GET /articles/:id/edit (edit)
  • PATCH/PUT /articles/:id (update)
  • DELETE /articles/:id (destroy)
  1. Named Routes:

Routes can be given names to make URL generation easier:

get 'profile', to: 'users#show', as: 'user_profile'

This allows you to use user_profile_path and user_profile_url helpers in your views and controllers.

  1. Nested Routes:

Routes can be nested to reflect hierarchical relationships between resources:

resources :authors do
  resources :books
end

This generates routes like GET /authors/:author_id/books.

Evaluating Responses:

  • The candidate should explain the basic principles of Rails routing and how routes are defined.
  • Look for an understanding of resourceful routing and its benefits.
  • A comprehensive answer should include examples of named routes and nested routes.

13. How would you optimize the performance of a Ruby on Rails application?

Question Explanation: Performance optimization is crucial for providing a good user experience and handling large-scale applications. This question assesses the candidate’s understanding of various techniques to enhance the performance of a Rails application.

Expected Answer: To optimize the performance of a Ruby on Rails application, several strategies can be employed:

  1. Database Optimization:
  • Indexing: Add indexes to database columns that are frequently queried. For example:
add_index :users, :email
  • Eager Loading: Use eager loading (includes) to reduce the number of database queries by loading associated records in advance.
  1. Caching:
  • Fragment Caching: Cache parts of views that are static or infrequently changed.
@users = User.includes(:posts).all
  • Page Caching: Cache entire pages to avoid hitting the Rails stack for static pages.
  • Action Caching: Cache the output of entire actions to serve subsequent requests quickly.
  • Low-level Caching: Use Rails’ low-level caching to cache arbitrary data.
<% cache @user do %>
  <%= render @user %>
<% end %>
  1. Asset Optimization:
    • Minification and Compression: Use tools to minify and compress JavaScript and CSS files.
    • Image Optimization: Use optimized image formats and tools to compress images.
  2. Background Jobs:
    • Offload long-running tasks to background jobs using tools like Sidekiq or ActiveJob, to free up web server resources for handling requests.
  3. Server and Network Optimization:
    • Use a CDN: Serve static assets via a Content Delivery Network (CDN) to reduce load times.
    • Optimize Web Server Configuration: Configure web servers (e.g., Nginx, Puma) for better performance.
  4. Code Optimization:
    • Profiling: Use profiling tools to identify and address bottlenecks in the code.
    • Memory Management: Optimize memory usage and reduce object allocation.

Evaluating Responses:

  • The candidate should mention various techniques such as database optimization, caching, asset optimization, background jobs, server/network optimization, and code optimization.
  • Look for specific examples and tools that can be used for each technique.
  • A strong answer should demonstrate an understanding of how these optimizations improve the overall performance of a Rails application.

14. What are partials in Ruby on Rails and how are they used?

Question Explanation:

Partials are a feature in Rails used for rendering reusable view templates. This question assesses the candidate’s knowledge of how to use partials to keep the view code DRY (Don’t Repeat Yourself).

Expected Answer:

Partials in Ruby on Rails are view templates that contain reusable code fragments. They are used to avoid duplication and make views more maintainable. Partials are typically named with a leading underscore and can be rendered within other views.

How to Use Partials:

  1. Creating a Partial:

Create a partial file with a leading underscore. For example, _user.html.erb might contain:

<div class="user">
  <h2><%= user.name %></h2>
  <p><%= user.email %></p>
</div>
  1. Rendering a Partial:

Render the partial in another view using the render method:

<%= render 'user', user: @user %>
  1. Collection Rendering:

Render a collection of items using a partial:

<%= render partial: 'user', collection: @users, as: :user %>
  1. Local Variables:

Pass local variables to a partial:

<%= render partial: 'user', locals: { user: @user } %>

Evaluating Responses:

  • The candidate should explain what partials are and their purpose in Rails views.
  • Look for an understanding of how to create and render partials, including passing local variables.
  • A good answer should include examples of rendering a single partial and a collection of partials.

15. How do you manage database transactions in Ruby on Rails?

Question Explanation:

Transactions are crucial for maintaining data integrity. This question assesses the candidate’s knowledge of how to use transactions in Rails to ensure atomic operations.

Expected Answer:

Database transactions in Ruby on Rails ensure that a series of operations are executed atomically, meaning either all operations succeed, or none are applied. This is useful for maintaining data integrity during complex operations.

Using Transactions in Rails:

  1. Manual Transactions:

Use the transaction method to wrap operations in a transaction block. If any operation within the block fails, all changes are rolled back.

ActiveRecord::Base.transaction do
  user.save!
  order.save!
end
  1. Automatic Rollbacks:

Rails automatically rolls back transactions if an exception is raised. The save! method (with the bang) raises an exception if the record is invalid, triggering a rollback.

begin
  ActiveRecord::Base.transaction do
    user.save!
    order.save!
  end
rescue ActiveRecord::RecordInvalid => e
  # Handle the error
end
  1. Nested Transactions:

Rails supports nested transactions using savepoints. However, it’s important to manage them carefully to avoid unexpected behavior.

ActiveRecord::Base.transaction do
  user.save!
  ActiveRecord::Base.transaction(requires_new: true) do
    order.save!
  end
end

Evaluating Responses:

  • The candidate should explain the concept of database transactions and their importance.
  • Look for examples of using the transaction method and handling exceptions to trigger rollbacks.
  • A good answer should include the use of nested transactions and potential pitfalls.

Ruby on Rails Interview Questions Conclusion

These Ruby on Rails interview questions and answers are designed to assess a candidate’s comprehensive understanding of the framework, covering fundamental concepts, advanced techniques, and best practices. By asking these questions, you can identify skilled engineers who are capable of building robust and scalable web applications, ensuring they can contribute effectively to your team.

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