Fixing N+1 queries when using `validates_associated` with `has_many`

validate_associated_n_queries.rb

# Blog post: https://veerpalbrar.github.io/blog/2021/11/26/Fix-N+1-queries-when-using-validates_associated-with-has_many 

### Setup
require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"
  gem "rails"
  gem "sqlite3"
end

require "active_record"
require "logger"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: "memory")
ActiveRecord::Base.logger = Logger.new(STDOUT)

Rails.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :authors, :force => true do |t|
    t.string :name
    t.integer :agent_id
  end

  create_table :books, :force => true do |t|
    t.integer :author_id
    t.string :title
  end

  create_table :covers, :force => true do |t|
    t.integer :book_id
  end
end

### Application Code
class Author < ActiveRecord::Base
  has_many :books
  validates_associated :books
  # Solution 2: Preload Books and Covers before validation
  # validate :books_are_valid

  # def books_are_valid
  #   books.preload(:cover).all?(&:valid?)
  # end
end

class Book < ActiveRecord::Base
  belongs_to :author
  has_one :cover
  validates :title, presence: true
  # solution 1: Only validate when neccessary
  validates_associated :cover #, if: -> { title_changed? }
end

class Cover < ActiveRecord::Base
  belongs_to :book
  validates_presence_of :book
end

a = Author.new(name: "First Last")
Book.create!(author: a, title: "Title2")
Cover.create!(book: Book.last)
Cover.create!(book: Book.create!(author: a, title: "Title"))
Cover.create!(book: Book.create!(author: a, title: "Title3"))

a = Author.last
a.name = "New Name"
a.save!