使用the Rails guides中的这个修改过的示例，一个如何使用mongoid对关系"has_many:through"关联进行建模？
挑战在于mongoid不像ActiveRecord那样支持has_many:through.
# doctor checking out patient
class Physician < ActiveRecord::Base
has_many :appointments
has_many :patients, :through => :appointments
has_many :meeting_notes, :through => :appointments
end
# notes taken during the appointment
class MeetingNote < ActiveRecord::Base
has_many :appointments
has_many :patients, :through => :appointments
has_many :physicians, :through => :appointments
end
# the patient
class Patient < ActiveRecord::Base
has_many :appointments
has_many :physicians, :through => :appointments
has_many :meeting_notes, :through => :appointments
end
# the appointment
class Appointment < ActiveRecord::Base
belongs_to :physician
belongs_to :patient
belongs_to :meeting_note
# has timestamp attribute
end