has_many :through with counter_cache

Question

It is my understanding that when defining a :counter_cache option it is to be specified on the model that includes the belongs_to declaration. So I am a little unsure of how

Answer 1

I'm not sure what kind of relationship you want. That example is similar to the one in the Rails Guide

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

• A Physician has many Appointments, and has many Patients
• An Appoinment belongs to (has one) Physician and one Patient
• a Patient has many Appointments and many Physicians.

Regarding the :counter_cache option, according to the belongs_to doc: If you want the number of Patients belonging to a Physician you would need:

class Appointment < ActiveRecord::Base
  belongs_to :physician, :counter_cache => :patient_count
  belongs_to :patient
end

And you need to write a migration to add the patient_count column to the Phyisicans table.

However, for has_many through relationships Rails 3.1 seems to automatically detect the counter_cache column, so you don't have to specify it (remove :counter_cache => :patient_count). If you do specify it your counter will go up by two (this is very weird).

By the way, there seems to be some problems with :counter_cache option in Rails 3.1, as reported here:

• https://github.com/rails/rails/issues/3903
• https://github.com/rails/rails/issues/3085

With all of that in mind, maybe your best bet is to write your own count mechanism using callbacks.

Hope it helps :)