eager_load.rb

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

Mongoid.configure.connect_to("mongoid_test")
logger = Logger.new $stdout
logger.level = 0
class Post
  include Mongoid::Document
  belongs_to :person
end

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

p = Person.create!(name: 'arthurnn')
Post.create!(person: p)
Post.create!(person: p)

Moped.logger = Mongoid.logger = logger

Mongoid::QueryCache.enabled = true

p Mongoid::QueryCache.enabled?
Person.includes(:posts).all.to_a
Person.includes(:posts).all.to_a
Person.includes(:posts).all.to_a
p Mongoid::QueryCache.cache_table