mongoid/mongoid — pull request #1625 by romanbsd (2012-01-22 10:47:03+00:00)
Title: Include _type in serialization

That fixes a problem with deserialization, which depends on this field.
I.e.

class Person
include Mongoid::Document
end

class Manager < Person
end

m = Manager.new
Person.new(m.serializable_hash).class.should eq(Manager)

I didn't figure out where to put a test like this.