Embedded document persisted w/o an id

photo.rb

class Photo
  include Mongoid::Document

  embedded_in :listing

  field :position, :type => Integer, :default => 0
end

widget.rb

class Widget
  include Mongoid::Document

  embeds_many :photos
end

widget_photos_log.txt

irb(main):002:0> w = Widget.find "4e371e3814c33c45d00002c2"
=> #<Widget _id: 4e371e3814c33c45d00002c2, serial_number: "001204484">

irb(main):003:0> w.photos 
=> [
#<Photo _id: 4e371e4714c33c45da000203, position: 1>, 
#<Photo _id: 4e371e4f14c33c45d00002c4, position: 4>, 
#<Photo _id: 4e371e5114c33c45da000205, position: 5>, 
#<Photo _id: 4e371e5314c33c45d00002c6, position: 6>, 
#<Photo _id: 4e371ec414c33c45d00002c8, position: 8>, 
#<Photo _id: 4e371ec714c33c45da000207, position: 9>, 
#<Photo _id: , position: 0>
]

irb(main):004:0> l.photos[6]
=> #<Photo _id: , position: 0>

irb(main):005:0> l.photos[6].destroy
=> true