Active Record (Ruby on Rails)

In Active Record a model class such as Book maps to a table by convention, with no configuration: the class name is pluralised to find the books table, the primary key is an integer id, foreign keys are named like author_id, and created_at and updated_at are filled in automatically. Columns become attributes automatically: an empty class Product < ActiveRecord::Base; end already has accessors for every column of products. Objects carry both data and behaviour: Book.find_by(title: ...) loads one, setting book.title and calling book.save writes an UPDATE, and validations and callbacks run around each save. Associations (belongs_to, has_many) are declared with one line each, and migrations describe schema changes in a Ruby DSL instead of SQL.

Associations load lazily, so iterating over 10 books and reading each author runs 11 queries; the Rails guide calls this the N + 1 queries problem. includes, preload and eager_load load associations up front, turning those 11 queries into 2, and strict_loading raises an error on any lazy load that slips through. When the query interface isn’t enough, find_by_sql runs your own SQL and still returns model objects. Active Record supports MySQL, MariaDB, PostgreSQL and SQLite and ships as the activerecord gem with every Rails release; Rails 8.1 is the current series.