class User
  include Mongoid::Document
  has_many :images
end

class Image
  include Mongoid::Document
  belongs_to :user
end

user = User.create
Image.create(:user => user)

image = Image.includes(:user).first
# MOPED: ... database=development collection=images selector={"$query"=>{}, "$orderby"=>{:_id=>1}} flags=[] limit=-1 skip=0 batch_size=nil fields=nil (3.6521ms)
# MOPED: ... database=development collection=users selector={"_id"=>{"$in"=>["514cb621f433ea9332000001"]}} flags=[] limit=0 skip=0 batch_size=nil fields=nil (0.2980ms)

image.user

# MOPED: ... database=development collection=users selector={"$query"=>{"_id"=>"514cb621f433ea9332000001"}, "$orderby"=>{:_id=>1}} flags=[] limit=-1 skip=0 batch_size=nil fields=nil (0.4160ms)

# Gemfile
GIT
  remote: git://github.com/mongoid/mongoid.git
  revision: de244e9c323d777cdf6c260e9461200ad932185b
  specs:
    mongoid (4.0.0)
      activemodel (~> 4.0.0.beta)
      moped (~> 1.4.2)
      origin (~> 1.0)
      tzinfo (~> 0.3.22)