benchmark.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'
    gem 'benchmark/ips'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

require 'active_record'
require 'minitest/autorun'
require 'logger'
require 'benchmark/ips'

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')

ActiveRecord::Schema.define do
  create_table :bs

  create_table :as do |t|
    t.references :b, index: true, foreign_key: true
  end
end

class A < ActiveRecord::Base
  belongs_to :b
end
class B < ActiveRecord::Base; end

A.create!

Benchmark.ips do |bm|
  bm.report do
    a = A.includes(:b).find_by_id(1)
    a.b
  end
end