Become a Rails Association Pro: Replicating has_one with Pure Ruby

3 min readApr 22
Become a Rails Association Pro: Replicating has_one with Pure Ruby


The ActiveRecord Object Relational Mapper (ORM) is one of its most potent features. This ORM offers a straightforward user interface for interfacing with relational databases, which is another one of its strengths. The ‘has_one’ association is extremely useful in Rails since it enables users to associate one record with another record. This makes it one of the most often used associations in the framework.

But what if you want to duplicate the functionality of has_one in pure Ruby without requiring Rails or ActiveRecord? In this post on the blog, we will be exploring how to successfully carry out the aforementioned task.

Understanding the has_one Association

First, let’s take a look back at what the has_one association does in Rails. Let’s say we have a “User” model that has_one “Profile” attached to it. This indicates that each record for a “User” has a single linked record for a “Profile”, and that each record for a “Profile” belongs to a single record for a “User”. Rails enable us to define this relationship in the following way,

class User < ApplicationRecord
has_one :profile

class Profile < ApplicationRecord
belongs_to :user

With this association in place, we can easily retrieve a user’s profile

user = User.find(1)
profile = user.profile

Rails takes care of building the proper SQL queries to obtain the corresponding record, and we do not need to worry about maintaining foreign keys or other low-level concerns because Rails handles all of that for us.


Rails provides many powerful tools for working with associations, it’s also possible to replicate the “has_one” association in pure Ruby code. This is helpful if you want to learn the inner workings of Rails associations or if you need to deal with associations outside of a Rails project.

To replicate the “has_one” association in pure Ruby, we’ll start by defining two classes: a “User” class and a “Profile” class. In our example, each user has one profile, and each profile belongs to one user.

# User class with id, name and…

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