Mongoid single object related to multiple fields

Is it possible to relate a single object as multiple fields? Something like this?

Ad Model

class Ad
  include Mongoid::Document
  field :name

  referenced_in :ad_types, :as => :web_spec
  referenced_in :ad_types, :as => :print_spec
end

AdType Model

class AdType
  include Mongoid::Document
  field :shape
  field :size
  field :medium

  references_many :ads
end

Then reference each reference as a separate field in a form like this.

<%= f.input :web_spec, :collection => AdType.where(:medium => "Web"), :label_method => :shape, :label => "Web" %>
<%= f.input :print_spec, :collection => AdType.where(:medium => "Print"), :label_method => :shape, :label => "Print" %>

I've given something like this with no luck. I'm probably going about it the wrong way or this feature just doesn't exist yet. Any suggestions would be great appeciated.

A few questions. Is there a reason why AdType is a document unto itself? Unless you're going to have a large number of types, it might make more sense for it to be an embedded document. Secondly, what might be cleaner is having a single association (regardless if it's referenced or embedded) and two scopes to your Ad model. Using your example, I'd have something like this:

class Ad
  include Mongoid::Document
  field :name
  embeds_one :ad_type

  scope :web ...
  scope :print ...
end

class AdType
  include Mongoid::Document
  field :shape
  field :size
  field :medium

  embedded_in :ad, :inverse_of => :ad_type
end