How can I join these tables?

A bit of a re-post, but I've cleaned up the question.

--MODELS--
class Unit < ActiveRecord::Base
  has_many :rooms

class Room < ActiveRecord::Base
  belongs_to :unit
  has_many :rentals
  has_many :people, :through => :rentals

class Person < ActiveRecord::Base
  has_many :rentals
  has_many :rooms, :through => :rentals

class Rental < ActiveRecord::Base
  belongs_to :room, :person

--QUESTION--
Using only the unit.id, how can I get the associated person.id?

Person.find_by_sql(...)

AR finders don't supplant SQL, think of them as convenience methods
for the simple cases..

Isak