Ruby on Rails 3: How can I sort ActiveRecords by an attribute of another table?

Question

I need to query a database table and get the rows ordered by a count of an association. Is there a Rails (like Active Record Query) way to do this?

My models and their associations are as follows:

class User < ActiveRecord::Base
has_one :business
end

class Business < ActiveRecord::Base
has_many :postulations
end

class Postulation < ActiveRecord::Base
belongs_to :business
end

I need to get a number of Users ordered by the amount of Postulations that their Business has. Is there a clean way to do this or do I just have to query with find_by_sql?

Thank you.

Answer 1

User.includes(:business => :postulations).group("users.id").order("count(postulations.id) desc").limit(20)

This will probably work