Sorry wasn't sure how to write the description, but given a class structure somewhat like this:

class UserIdentifier
  include Mongoid::Document
  class Facebook < UserIdentifier; end
  class Email < UserIdentifier
     field :subscribed, type: Boolean, default: true
  end
end

UserIdentifier::Facebook.create.as_document #=> {"subscribed" => true}

Is the fact that the field was added to a different class yet applied to all siblings/parents a bug? It definitely goes against my expectations if it's working as designed.