Question

How can I write an AR find query to have the results ordered by the number of records in a has_many association?

class User < ActiveRecord::Base
  has_many :photos
end

I want to do something like…

User.find(:all, :order => photos.count)

I realize my find is not valid code. Say I have the following data.

User 1, which has 3 photos 
User 2, which has 5 photos 
User 3, which has 2 photos 

I want my find to bring me back the users in the order of…

User 2, 
User 1, 
User 3 

based on the count of of the users photos

Answer 1

If you don’t want an extra column, you could always ask for an extra column in the returned result set:

User.all(:select => "#{User.table_name}.*, COUNT(#{Photo.table_name}.id) number_of_photos",
         :joins => :photos,
         :order => "number_of_photos")

This generates the following SQL:

SELECT users.*, COUNT(photos.id) number_of_photos
FROM `users` INNER JOIN `photos` ON photos.user_id = users.id
ORDER BY number_of_photos