class Parent
  include Mongoid::Document
  include Mongoid::Timestamps

  field :name

  embeds_one :child
end

class Child
  include Mongoid::Document
  include Mongoid::Timestamps

  field :name

  embedded_in :parent
end

1.9.3p194 :004 > Parent.delete_all
 => 1 
1.9.3p194 :005 > Parent.create(child: Child.new)
 => #<Parent _id: 51267e99499c194060000003, created_at: 2013-02-21 20:07:53 UTC, updated_at: 2013-02-21 20:07:53 UTC, name: nil> 
1.9.3p194 :006 > u = Parent.first.updated_at
 => Thu, 21 Feb 2013 20:07:53 UTC +00:00 
1.9.3p194 :007 > c = Parent.first.child
 => #<Child _id: 51267e99499c194060000004, created_at: nil, updated_at: nil, name: nil> 
1.9.3p194 :008 > c.name = "Touch the parent"
 => "Touch the parent" 
1.9.3p194 :009 > c.save
 => true 
1.9.3p194 :010 > u == Parent.first.updated_at
 => true 
1.9.3p194 :011 > Parent.first
 => #<Parent _id: 51267e99499c194060000003, created_at: 2013-02-21 20:07:53 UTC, updated_at: 2013-02-21 20:07:53 UTC, name: nil> 
1.9.3p194 :013 > Parent.first.child
 => #<Child _id: 51267e99499c194060000004, created_at: nil, updated_at: 2013-02-21 20:08:12 UTC, name: "Touch the parent">