For http://stackoverflow.com/q/14475862/693538

gistfile1.rb

# First I modify your models a bit:

Class Appointment ...
  # belongs_to :doctor   (replace it with this ↓)
  has_one :doctor, through: :schedule # This means 'doctor_id' is not stored in appointments table, it is got from schedule via join
  belongs_to :schedule
end

Class Schedule ...
  belongs_to :doctor
  # has_many :appointments   (add this ↓)
  has_many :appointments, dependent: :destroy
end

Class Doctor ...
  # has_many :appointments, dependent: :destroy   (replace with this ↓, dependent: moved to Schedule)
  has_many :appointments, through: :schedule
  has_many :schedules, dependent: :destroy
end

# -----
# Now assume that we are in the view schedules/show and
# we have @schedule set to the current schedule (in the controller):
@schedule  # => returns current schedule we a showing

# You know that this:
@schedule.appointments
# ...will return the list of appointments in GIVEN schedule.

# But it is not a simple array, it is an _association_ and it has methods:
a = @schedule.appointments.build
#...returns a new appointment ASSOCIATED with current @schedule, that is, it has its schedule_id set.
# I.e, it is the same as
a = Appointment.new :schedule => @schedule

# You can check that:
a.schedule # will return @schedule
a.doctor # will return @schedule.doctor: that is how :through association works

# You see, this new appointment has no info in it, but it has two correct associations!
# So if you have your route for appointments#new is like
# new_doctor_appointment GET    /:locale/doctors/:doctor_id/schedules/:schedule_id/appointments/new
# You can create a link to a appointments#new page with
a = @schedule.appointments.build
link_to "New appointment", new_doctor_appointment_path(a, locale: 'en')
# ... and it will get doctor_id and schedule_id from `a` object.

# Note that in order to make the routes look like this, you have to replace
  resources :doctors do
    resources :appointments  
    resources :schedules  
  end
# with
  resources :doctors do
    resources :schedules do
      resources :appointments  
    end
  end
# If I am right and appointments are nested in schedules, than it's better to show this in routes too.

NIA, thank you for the comments, I'm grasping the concepts. But I'm a bit lost in the controllers.

How do I pass the parameters to the doctor_controller index action?

a) @DoctorS = Doctor.all
b) @schedule = Schedule.find(params[:doctor_id] @DoctorS = @schedule.doctors.all
c) ....

In the new action I'm going to replace @doctor = Doctor.new with @doctor = schedule.build?

this is the repo: https://github.com/jmercedes/pgranges/tree/refactorcode

Wait, wait. Schedules belong to Doctor, not vice versa. You somehow reverse this, it is wrong.

That's why no modification needed in doctors_controller. It is the root of current ierarchy and it remains unchanged. @doctor = Doctor.all is ok for index action, @doctor = Doctor.new is ok for new action — no changes.

I looked your repo and see that schedules_controller is also ok. Indeed, it should not change when we move appointments inside of schedules.

You only have to modify your Appoitments controller. I will post my comments about it to the commit in your repository