demo.rb

#!/usr/bin/env ruby
# frozen_string_literal: true

require 'active_record'
require 'logger'
require 'sqlite3'
require 'query_count'

require_relative 'setup'

# Something to note: ActiveRecord is typically used to
# retrieve whole objects, not just attributes. This is
# because it is an ORM (Object Relational Mapper).
# This is not always efficient.

# Define post model
class Post < ApplicationRecord
  has_many :comments

  # Page 5, the `.last` executes the query,
  # so it's not efficient. This doesn't matter
  # whether there is ain `includes` to the left,
  # it still produces n+1 queries.
  def latest_comment
    comments.order(:created_at).last
  end
end

# Define comment model
class Comment < ApplicationRecord
  belongs_to :post
end

# Seeding Data
def seed_one_post
  post = Post.create(title: 'Example Post 1')
  id = post.id
  post.comments.create(body: 'This is a comment')
  post.comments.create(body: 'This is another comment')

  (1..10).each do |i|
    post.comments.create(body: "Comment #{i}")
  end

  id
end

def seed_two_posts
  count = 1
  post = Post.create(title: 'Example Post 1')
  # id = post.id
  # post.acomments.create(body: 'This is a comment')
  # post.comments.create(body: 'This is another comment')
  (0..count).each do |i|
    post.comments.create(body: "Comment #{i}")
  end

  post = Post.create(title: 'Example Post 2')
  (0..count).each do |i|
    post.comments.create(body: "Comment #{i}")
  end
end

def seed(post_count:, comment_count:)
  (1..post_count).each do |i|
    post = Post.create(title: "Example Post #{i}")
    (1..comment_count).each do |j|
      post.comments.create(body: "Comment #{j}")
    end
  end
end

def with_includes
  # Querying Data

  post_id = post.id # Use the ID of the post you want to retrieve

  # This is 2 database calls.
  # queried_post = Post.includes(:comments).find(post_id)

  queried_post = Post.eager_load(:comments).find(post_id)

  # binding.irb

  # This doesn't work because the comments are not preloaded.
  # queried_post = Post.find(post_id).includes(:comments)

  puts "Post: #{queried_post.title}"
  queried_post.comments.each do |comment|
    puts " - Comment: #{comment.body}"
  end
rescue ActiveRecord::RecordNotFound
  puts "Post with id #{post_id} not found."
end

def without_includes(post_id)
  queried_post = Post.find(post_id)

  # binding.irb

  # This doesn't work because the comments are not preloaded.
  # queried_post = Post.find(post_id)

  puts "Post: #{queried_post.title}"
  queried_post.comments.each do |comment|
    puts " - Comment: #{comment.body}"
  end
rescue ActiveRecord::RecordNotFound
  puts "Post with id #{post_id} not found."
end

# This does not do n+1
# post_id = seed_one_post
# without_includes(post_id)

# post_id = seed_two_posts

# Run this ./demo > tmp/out.txt
# Check with grep "SELECT" tmp/out.txt | wc -l
# Post.limit(1000).find_each do |post|
#     post.comments.each do |comment|
#     puts comment.body
#   end
# end

# Page 4.
seed(post_count: 1, comment_count: 1)
ActiveRecord::Base.logger = Logger.new($stdout)
# The `includes` is ignored here.
Post.includes(:comments).find_each do |post|
  # But here rails will "ignore" your includes and run a query
  # for each post (n+1 queries)
  puts post.latest_comment
end