# Advanced ActiveRecord Queries (Ruby on Rails)

> Query interface, where, joins, includes, eager loading, N+1 problem, transactions, locking (optimistic/pessimistic)

- 28 interview questions
- Mid-Level
- [Interview Questions: Ruby on Rails](https://sharpskill.dev/en/technologies/ruby-on-rails/interview-questions.md)

## 1. Which ActiveRecord method retrieves a record by its primary key and raises an exception if not found?

**Answer**

The find method retrieves a record by its primary key and raises an ActiveRecord::RecordNotFound exception if the record does not exist. This ensures the code fails explicitly rather than continuing with a nil value. To avoid the exception and return nil instead, use find_by(id: value).

## 2. What is the main difference between find_by and where.first in ActiveRecord?

**Answer**

find_by directly returns a single record or nil, while where.first returns an ActiveRecord relation then calls first to get the first result. In practice, find_by generates an optimized query with LIMIT 1, whereas where can potentially load more data before selecting the first element.

## 3. How to pass multiple conditions to the where method in ActiveRecord?

**Answer**

The where method accepts a hash with multiple keys to specify multiple conditions that will be combined with AND. For example, User.where(status: 'active', role: 'admin') generates WHERE status = 'active' AND role = 'admin'. For OR conditions, use or or raw SQL queries.

## 25 more questions available

- What is the N+1 problem in ActiveRecord?
- Which method to use to solve the N+1 problem by preloading associations?

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