Left joining in RoR

README.md

Left joins

Given 3 models: User, Book, Reading. How to return a list of all books and mark the ones read by a given user.

The result could look something like:

[
  {
    "id": 1,
    "title": "To Kill a Mockingbird",
    "read": false
  },
  {
    "id": 2,
    "title": "East of Eden",
    "read": true
  },
  {
    "id": 3,
    "title": "On the Road",
    "read": false
  }
]

Where read indicates there is a Reading object linking this Book to this User

book.rb

class Book < ApplicationRecord
  has_many :readings, dependent: :destroy
  
  validates :title, presence: true
end

book_serializer.rb

class BookSerializer < ActiveModel::Serializer
  attributes :id, :title, :read

  def read
    object.read
  end
end

query.sql

SELECT books.*, CASE WHEN readings.id IS NULL THEN false ELSE true END as read
FROM "books" 
LEFT OUTER JOIN "readings" ON "readings"."book_id" = "books"."id" and user_id = '<some id>';

readings.rb

class Reading < ApplicationRecord
  belongs_to :user
  belongs_to :book
end

user.rb

class User < ApplicationRecord
  has_many :readings, dependent: :destroy
  
  validates :name, present: true
end

user_controller.rb

class UsersController < ApplicationController
  def reading_list
    books = Book
      .select('books.*, CASE WHEN readings.id IS NULL THEN false ELSE true END as read')
      .joins("LEFT OUTER JOIN readings ON reagins.book_id = books.id AND readings.user_id = '#{current_user.id}'")
      .all
    render json: books
  end
end