gistfile1.rb

  scope :ordered_by_article_count,
    select("users.*, c.count as article_count").
      joins("LEFT JOIN (SELECT user_id, COUNT(user_id) as count
              FROM articles WHERE state = 'active' GROUP BY user_id) as c ON c.user_id = users.id").
        where("c.count IS NOT NULL").
          order("c.count DESC")