How to sort Rails AR.find by number of objects in a has_many relationship

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_man         


        

Answer 1

I'd add this as a comment on the top answer, but I can't for some reason. According to this post:

http://m.onkey.org/active-record-query-interface

The User.all(options) method will be deprecated after Rails 3.0.3, and replaced with a bunch of other (handy, chainable) active record type stuff, but it makes it very hard to figure out how to put together the same kind of query.

As a result, I've gone ahead and implemented the counter cache method. This was pretty easy and painless with the exception that you need to remember to update the column information in your migration, otherwise all existing records will have "0."

Here's what I used in my migration:

class AddUserCountToCollections < ActiveRecord::Migration
  def self.up
    add_column :collections, :collectionusers_count, :integer, :default => 0
    Collection.reset_column_information
    Collection.all.each do |c| 
      Collection.update_counters c.id, :collectionusers_count => c.collectionusers.count
    end
  end

  def self.down
    remove_column :collections, :collectionusers_count
  end
end

In theory this should be faster, too. Hope that's helpful going forward.