has_one relationship in Ruby on Rails

Introduction

The has_one relationship links two models so one model owns exactly one related model. It helps organize data clearly.

When a user has exactly one profile.

When a car has exactly one engine.

When a company has exactly one headquarters address.

When an order has exactly one payment detail.

Syntax

Ruby on Rails

class User < ApplicationRecord
  has_one :profile
end

Use has_one in the model that owns the other model.

The related model usually has a foreign key pointing back.

Examples

User has one Profile, and Profile belongs to User. This sets up a one-to-one link.

Ruby on Rails

class User < ApplicationRecord
  has_one :profile
end

class Profile < ApplicationRecord
  belongs_to :user
end

Each Car has one Engine. Engine stores the car_id foreign key.

Ruby on Rails

class Car < ApplicationRecord
  has_one :engine
end

class Engine < ApplicationRecord
  belongs_to :car
end

Sample Program

This example shows an Author with one Biography. We create both and then print the biography content through the author.

Ruby on Rails

class Author < ApplicationRecord
  has_one :biography
end

class Biography < ApplicationRecord
  belongs_to :author
end

# Usage in Rails console
author = Author.create(name: "Jane Doe")
biography = Biography.create(content: "Writer and traveler.", author: author)

puts author.biography.content

Important Notes

Always add belongs_to on the related model for proper linking.

Use database foreign keys to keep data consistent.

Access the related object easily via the has_one association.

Summary

has_one sets a one-to-one connection from one model to another.

The related model uses belongs_to to link back.

This helps keep data organized and easy to access.