rails 4 has_many :through、has_one :through和Has_many :through, :source

has_many :through

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