posts =  Post.all.includes(:person)

require 'mongoid'
require 'mongoid/support/query_counter'
require 'minitest/autorun'
# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

Mongoid.configure.connect_to("mongoid_test")

Mongoid.logger = Logger.new($stdout)
Moped.logger = Logger.new($stdout)

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

class Post
  include Mongoid::Document
  belongs_to :person
end


class Person
  include Mongoid::Document
  has_many :posts
  field :name
end

class BugTest < Minitest::Test
  def test_query_count
    p = Person.create!(name: 'arthurnn')
    4.times { Post.create!(person: p) }

    criteria =  Post.all.includes(:person)

    query_counter = count_queries do
      criteria.each(&:person)
    end

    # this is fine
    assert_equal 2, query_counter

    query_counter = count_queries do
      criteria.each(&:person)
    end

    # this fails
    assert_equal 2, query_counter
  end
end

Finished tests in 0.114663s, 8.7212 tests/s, 17.4424 assertions/s.

  1) Failure:
BugTest#test_query_count [examples/mongoid_test.rb:49]:
Expected: 2
  Actual: 43

1 tests, 2 assertions, 1 failures, 0 errors, 0 skips