ActiveRecord, has_many :through, Polymorphic Associations with STI

In ActiveRecord, has_many :through, and Polymorphic Associations, the OP's example requests ignoring the possible superclass of Alien and Person (SentientBeing). This is where my question lies.

class Widget < ActiveRecord::Base
  has_many :widget_groupings

  has_many :people, :through => :widget_groupings, :source => :person, :source_type => 'Person'
  has_many :aliens, :through => :widget_groupings, :source => :alien, :source_type => 'Alien'
end

SentientBeing < ActiveRecord::Base
  has_many :widget_groupings, :as => grouper
  has_many :widgets, :through => :widget_groupings
end


class Person < SentientBeing
end

class Alien < SentientBeing
end

In this modified example the grouper_type value for Alien and Person are now both stored by Rails as SentientBeing (Rails seeks out the base class for this grouper_type value).

What is the proper way to modify the has_many's in Widget to filter by type in such a case? I want to be able to do Widget.find(n).people and Widget.find(n).aliens, but currently both of these methods (.people and .aliens) return empty set [] because grouper_type is always SentientBeing.