custom serializer to handle root and embedded documents alike in the association. WARNING: behaves like a field rather than an association

shallow_document.rb

##
# class Activity
#   include Mongoid::Document
#   
#   field :trackable, type: ShallowDocument
# end
##

class ShallowDocument
  include Mongoid::Fields::Serializable

  def deserialize(object={})
    return nil if object.blank?
    result = (object['klass'].constantize).where(_id: object['_id']).first

    (object['selector'] || []).each do |message|
      result = result.send(*message)
    end if result

    result
  end

  def serialize(object)
    return object unless object.is_a?(::Mongoid::Document)
    attr = {}
    root = object._root

    attr.store('klass', root.class.to_s)
    attr.store('_id', root._id.to_s)
    attr.store('selector', object.selector_from(root)) if object.embedded?
    attr
  end
end