class Widget
include Mongoid::Document

identity :type => Integer
field :name, :type => String
end

Widget.new :_id => 5, :name => "Fred" #=> #<Widget _id: 5, _type: nil, _id: 5, name: "Fred">

As you can see, the _id field appears a second time, and the _type field appears even though there's no inheritance going on. This is on 2.0.0.rc.8.