HABTM Preload Benchmark

gistfile1.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'arel', github: 'rails/arel'
    gem 'sqlite3'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

require 'active_record'
require 'logger'

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
#ActiveRecord::Base.logger = Logger.new(STDOUT)
ActiveRecord::Base.logger = Logger.new(File.open("/dev/null", 'w'))

ActiveRecord::Schema.define do
  create_table :library_entries do |t|
    t.integer :book_id
  end

  create_table :books do |t|
  end

  create_table :genres do |t|
  end

  create_table :books_genres, id: false do |t|
    t.integer :book_id
    t.integer :genre_id
  end
end

class Book < ActiveRecord::Base
  has_and_belongs_to_many :genres
end

class Genre < ActiveRecord::Base
  has_and_belongs_to_many :books
end

class LibraryEntry < ActiveRecord::Base
  belongs_to :book
end

puts "Initializing database."

books = []
genres = []
library_entries = []
ActiveRecord::Base.transaction do
  genres = (1..20).map { Genre.create! }
  books = (1..5000).map do
    book = Book.new
    book.genres = genres.sample(5)
    book.save!
    book
  end
  library_entries = (1..5000).map do |i|
    l = LibraryEntry.new
    l.book = books[i]
    l.save!
    l
  end
end

puts "Finished initializing database."

test_preload = true

Benchmark.bm do |x|
  1.upto(25) do |i|
    c = i * 20
    if test_preload
      x.report("Preload: #{c}") { LibraryEntry.limit(c).includes(:book, book: :genres).to_a }
    else
      x.report("n+1:     #{c}") { LibraryEntry.limit(c).includes(:book).map {|x| x.book.genres.to_a } }
    end
  end
end