Benchmark used for HABTM performance comparison

perf.rb

require_relative '../load_paths'
require 'active_record'

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')

class Book < ActiveRecord::Base
  has_and_belongs_to_many :genres
  has_many :genre_memberships
  has_many :genres2, through: :genre_memberships, source: :genre
end

class Genre < ActiveRecord::Base
  has_and_belongs_to_many :books
  has_many :genre_memberships
  has_many :books2, through: :genre_memberships, source: :book
end

class LibraryEntry < ActiveRecord::Base
  belongs_to :book
end

class GenreMembership < ActiveRecord::Base
  belongs_to :book
  belongs_to :genre
end

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

  create_table :genre_memberships do |t|
    t.integer :book_id
    t.integer :genre_id
  end
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
    g = genres.sample(5)
    book.genres = g
    book.genres2 = g
    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."

require 'benchmark/ips'
Benchmark.ips do |bm|
  bm.report('no id') { LibraryEntry.limit(200).includes(book: :genres).to_a }
  bm.report('with id') { LibraryEntry.limit(200).includes(book: :genres2).to_a }
end