class Product
  include Mongoid::Document
  include Mongoid::Timestamps

  field :name

  embeds_one :seo, as: :seo_tags, cascade_callbacks: true, autobuild: true
  accepts_nested_attributes_for :seo, reject_if: :all_blank
end

class Seo
  include Mongoid::Document
  include Mongoid::Timestamps

  field :title

  embedded_in :seo_tags, polymorphic: true
end

irb(main):001:0> p = Product.create name: "Hello"
=> #<Product _id: 511386d0c185641c9c000002, name: "Hello", created_at: 2013-02-07 10:49:52 UTC, updated_at: 2013-02-07 10:49:52 UTC>
irb(main):002:0> p.seo
=> #<Seo _id: 511386d0c185641c9c000001, _type: nil, title: nil, keywords: nil, description: nil, noindex: nil, nofollow: nil>
irb(main):003:0> p.seo.persisted?
=> true