class Person
include Mongoid::Document
embeds_one :address
end

class Address
include Mongoid::Document
embedded_in :addressable, :inverse_of => :address
end

p = Person.new #=> #<Person _id: 4d7151bfd4ec9e673d000001, >
p.address = Address.new
p.address => #<Address _id: 4d7151d0d4ec9e673d000003, >
p.clone.address #=> nil
p.save! #=> true
p.clone.address => #<Address _id: 4d7151d0d4ec9e673d000003, >