How to write a "scope" on a join model?

Suppose I have these models: a Physician has many Patients through their Appointments.

class Physician < ActiveRecord::Base
  has_many :appointments
  has_many :patients, :through => :appointments
end
 
class Appointment < ActiveRecord::Base
  belongs_to :physician
  belongs_to :patient
end
 
class Patient < ActiveRecord::Base
  has_many :appointments
  has_many :physicians, :through => :appointments
end

I want to write a scope or something similar so that I can find all Patients of a given Physician whose Appointments are confirmed.

What is the most idiomatic 开发者_开发知识库way to do this?

To do this with a has_many :through association:

has_many :confirmed_patients, :through => :appointments, :source => :patient, :class_name => 'Patient', :conditions => { :confirmed => true }