by latest two versions, '4.0.0.beta1' behaves much better than '3.1.6'

n+1_benchmark_mongoid.rb

#!/usr/bin/env ruby

# encoding: utf-8

# ======================================================
# <N+1 problem>
#
# Post.all.each do |p|
#   puts p.category.name
# end
#
# select * from POSTS;
#
# select * from CATEGORYS where POST_ID=1;
# select * from CATEGORYS where POST_ID=2;
# select * from CATEGORYS where POST_ID=3;
# select * from CATEGORYS where POST_ID=4;
#
# --- solution ---
#
# Post.all.includes(:category).each do |p|
#   puts p.category.name
# end
#
# select * from POSTS;
#
# select CATEGORY.* from CATEGORY
#   where CATEGORY.POST_ID IN (1,2,3,4)
#
# ======================================================

require 'rubygems'
require 'faker'

# ====================================
# comment to switch mongoid version
# result in different performance
#
gem 'mongoid', "3.1.6"
# gem 'mongoid', "4.0.0.beta1"
# ====================================

require 'mongoid'
require 'benchmark'

# This call creates a connection to our database.
Mongoid::Config.connect_to('test')

# Mongoid::Config.identity_map_enabled = true

# First, set up our database...
class Category
  include Mongoid::Document
  has_many :items
  field :name
end

class Item
  include Mongoid::Document
  belongs_to :category
end

Category.create(:name=>"Sara Campbell Stuff")
Category.create(:name=>"Jake Moran Possessions")
Category.create(:name=>"Josh Items")
number_of_categories = Category.count

puts "Loading data..."

item_count = Item.count
item_table_size = 10000

if item_count < item_table_size
  (item_table_size - item_count).times do
    Item.create!(:name=>Faker.name,
                 :category_id=>(1+rand(number_of_categories.to_i)))
  end
end

puts "Running tests..."

Benchmark.bm do |x|
  [100,1000,10000].each do |size|
    x.report "size:#{size}, with n+1 problem" do
      @items=Item.all.limit(size)
      @items.each do |i|
        i.category
      end
    end
    x.report "size:#{size}, with :include" do
      @items=Item.all.includes(:category).limit(size)
      @items.each do |i|
        i.category
      end
    end
  end
end