A bit of a re-post, but I've cleaned up the question.
class Unit < ActiveRecord::Base
class Room < ActiveRecord::Base
has_many :people, :through => :rentals
class Person < ActiveRecord::Base
has_many :rooms, :through => :rentals
class Rental < ActiveRecord::Base
belongs_to :room, :person
Using only the unit.id, how can I get the associated person.id?
AR finders don't supplant SQL, think of them as convenience methods
for the simple cases..