Ruby on Rails offers many tools to link models in your database, sometimes so many that it can be confusing to find the correct solution for your problem. I recently had two models, User and Image, that joined on two tables, Reviews and Favorites. Finding the right solution for this issue took some leg work, but eventually I implemented a clean solution that worked just as I needed it to. Keep reading if you too want to discover how to handle models that join on multiple tables.

The problem

I made a web app that had Users and Images. Users and Images both had many favorites and many reviews. Favorites are created when a user favorites an image, and reviews are created when a user reviews an image.

I needed both my users to both access what images they have liked and what images they have favorited, and the reciprocal for my images.

If I only had one joins table, I would achieve this relationship with the following code:

class User < ApplicationRecord
has_many :images, through: :reviews

However, things start to look wrong when you add in the second relationship:

has_many :images, through: :reviews
has_many :images, through: :favorites

The solution

Solving the above problem required me to search in a variety of places and was not the easiest to find, however the solution itself is very simple.

has_many :reviewed_images, :through => :reviews, :source => :image
has_many :favorite_images, :through => :favorites, :source => :image

This allows me to access the necessary data easily.

Usage

Let's say I was playing around with this code in my Ruby console. If I set an instance of my user class equal to u1, I could call that user's favorite images with the syntax u1.favorite_images, and I could call their reviewed images with u1.reviewed_images. Pairing this access with my Controllers and Serializers, we can now send the nested data to the front end so our app operates the way we want it to.