mongoid by "4.0.0.beta1", https://github.com/mongoid/mongoid/blob/master/examples/mongoid_test.rb

n+1-mongoid_query_count.rb

#!/usr/bin/env ruby

require 'mongoid'
require 'mongoid/support/query_counter'

Mongoid.configure.connect_to("mongoid_test")

def count_queries(&block)
  query_counter = Mongoid::QueryCounter.new
  query_counter.instrument(&block)
  query_counter.events.size
end

class Post
  include Mongoid::Document
  has_many :comments
end

class Comment
  include Mongoid::Document
  belongs_to :post
  field :date
end

Post.delete_all
Comment.delete_all

post1 = Post.create!
post2 = Post.create!
post3 = Post.create!

post1.comments << Comment.create!(:date => 'today')
post1.comments << Comment.create!(:date => 'today')
post2.comments << Comment.create!(:date => 'today')
post2.comments << Comment.create!(:date => 'today')
post2.comments << Comment.create!(:date => 'today')

query_counter1 = count_queries do
  Post.each do |post|
    post.comments.map(&:date)
  end
end

puts "without incldues: #{query_counter1}"
# => 4

query_counter2 = count_queries do
  Post.includes(:comments).each do |post|
    post.comments.map(&:date)
  end
end

puts "with includes: #{query_counter2}"
# => 2