30 Rails Interview Questions and Answers

Ruby on Rails (Rails) is an open-source web application framework written in Ruby that follows the MVC architectural pattern. It emphasizes Convention over Configuration (CoC) and Don't Repeat Yourself (DRY) principles, making web development faster and more efficient.

In Rails, MVC (Model-View-Controller) separates application logic: Models handle data and business logic, Views handle presentation, and Controllers handle user requests and coordinate between Models and Views. This separation makes code organized and maintainable.

Active Record is Rails' ORM (Object-Relational Mapping) that connects objects to database tables. It provides an interface for database operations, handles validations, associations, and migrations, making database interactions more Ruby-like and intuitive.

Gems are packaged Ruby libraries or applications that add functionality to Rails applications. They can be installed using the RubyGems package manager. Common gems include devise (authentication), cancancan (authorization), and rspec (testing).

Rails routing determines how URLs map to controller actions. Routes are defined in config/routes.rb and follow RESTful conventions. They specify which controller and action should handle specific HTTP requests, making URL handling organized and predictable.

Migrations are Ruby classes that make it easy to modify your database schema over time. They allow version control of database changes, making it simple to modify tables, add columns, or roll back changes in a structured way.

Rails uses Asset Pipeline (Sprockets) to combine, minify, and compress JavaScript, CSS, and image files. It helps manage application assets, improves load times, and supports preprocessing of assets using tools like Sass or CoffeeScript.

Partials are reusable view components in Rails, denoted by an underscore prefix (_form.html.erb). They help DRY up views by allowing common HTML elements to be shared across multiple pages, improving maintainability.

Rails helpers are modules containing methods that assist view templates. They help keep views clean by moving complex logic into helper methods. Common helpers include form_for, link_to, and custom helpers for specific functionality.

Scaffolding is a Rails feature that automatically generates a set of models, views, controllers, and database migrations for a resource. It's useful for rapid prototyping but typically modified for production applications.

Concerns in Rails are modules that allow you to share common code between multiple models or controllers. They help implement DRY principle by extracting reusable code into modules with extend ActiveSupport::Concern. They're particularly useful for cross-cutting functionality like authentication logic or timestamp handling.

Authentication can be implemented using has_secure_password in the User model, which adds password encryption via bcrypt. You'll need to create sessions controller for login/logout, add password_digest column, handle user registration, manage sessions using cookies/session hash, and implement authentication filters in controllers.

before_save runs before both create and update operations, while before_create runs only when creating new records. Both are Active Record callbacks, but before_save is more general. For example, before_save might format data, while before_create might set initial values for new records only.

Rails supports multiple caching strategies: page caching (entire pages), action caching (with filters), fragment caching (partial content), and low-level caching. It uses cache stores like Redis or Memcached, and includes features like Russian Doll caching and cache key versioning for efficient cache invalidation.

Service objects encapsulate complex business logic that doesn't belong in models or controllers. They follow Single Responsibility Principle and are used for operations like complex user registrations, API interactions, or multi-step processes. They keep models focused on data and controllers on request handling.

Strong parameters provide security against mass assignment vulnerabilities by requiring explicit specification of allowed parameters in controllers. Using permit and require methods, they filter out unauthorized parameters from request parameters before they reach model creation or update actions.

includes performs eager loading to prevent N+1 queries by loading associated records in advance, using LEFT OUTER JOIN. joins performs INNER JOIN and is useful for filtering based on associations. includes is for optimization, while joins is primarily for querying related data.

Background jobs in Rails are typically handled using Active Job framework with backends like Sidekiq or Delayed Job. They're used for time-consuming tasks like email sending, file processing, or API calls. Jobs can be scheduled, retried, and monitored, improving application responsiveness.

Polymorphic associations allow a model to belong to multiple models through a single association. They're implemented using type and id columns, useful for features like comments or likes that can belong to different types of records. They provide flexible and maintainable ways to handle complex relationships.

Concerns are specialized modules that extend ActiveSupport::Concern, providing proper module inclusion handling and dependent loading. Unlike traditional modules, they handle class methods properly, manage dependencies between concerns, and are specifically designed for Rails' modular code organization.

The Asset Pipeline (Sprockets) historically managed static assets, while Webpacker integrates modern JavaScript tools. Rails 6+ uses both: Asset Pipeline for CSS/images and Webpacker for JavaScript/NPM packages. Webpacker compiles JavaScript using Webpack, supports ES6, and manages node_modules. Choose based on needs: Asset Pipeline for simpler apps, Webpacker for complex JavaScript requirements.

Performance optimization involves multiple strategies: implementing proper database indexing, using counter_cache for counting associations, leveraging Russian Doll caching, using bullet gem to detect N+1 queries, implementing background jobs for heavy tasks, using rack-mini-profiler for bottleneck detection, and considering horizontal scaling through load balancing.

Rails is thread-safe since version 2.2, but requires careful consideration. Thread safety involves class-level caching, global variables, and mutable objects. Use Thread.current for thread-local variables, ensure gem compatibility, and understand how ActiveRecord connection pool works. Important for handling concurrent requests in production environments.

Large-scale testing involves multiple layers: unit tests with RSpec for models, controller tests for API endpoints, integration tests for critical workflows, and system tests with Capybara for UI. Use factories (FactoryBot) over fixtures, implement test parallelization, mock external services, and maintain test isolation for faster execution.

Beyond basic gems like CanCanCan, complex authorization might require: role-based access control (RBAC) with hierarchical permissions, policy objects using Pundit for contextual authorization, custom authorization rules in services, caching permission checks, and integration with external identity providers or SSO systems.

Real-time features can be implemented using Action Cable for WebSocket connections, integrating with Redis for pub/sub messaging, handling connection authentication, scaling WebSocket connections across multiple servers, and implementing proper error handling and reconnection strategies. Consider alternatives like HTTP streaming or Server-Sent Events.

Large applications require careful schema design: using database constraints and indexes effectively, implementing partitioning for large tables, considering polymorphic associations vs. Single Table Inheritance, managing complex relationships, implementing soft deletes, and planning for future scalability through careful migration management.

Metaprogramming in Rails involves dynamic method generation using define_method, method_missing for ghost methods, using class_eval and instance_eval for runtime code evaluation, extending classes dynamically, and creating DSLs. Common in Rails internals like has_many associations or scope definitions.

Implementing microservices involves: breaking monoliths into smaller services, using API versioning, implementing service discovery, managing inter-service communication (REST/gRPC), handling distributed transactions, implementing circuit breakers (Hystrix), and maintaining service documentation. Consider using Rails API mode for lighter services.

Large data migrations require: batching updates using find_each, implementing reversible migrations, using background jobs for processing, maintaining zero-downtime deployments, implementing data validation and rollback strategies, monitoring performance impact, and possibly using tools like strong_migrations gem for safe operations.