Order by count of active record relations

example.rb

class Topic < ActiveRecord::Base
  has_many :votes
  
  # Something that provides the result of the method below,
  # but without dominating the db
  #SQL would look something like this
  #   SELECT topics.*, COUNT(id) as vote_count from topics
  #   join on votes where topic_id = topics.id
  #   group_by(topic_id)
  #   order by (vote_count)
  
  def self.order_by_count
    Topic.sort_by { |topic| 1.0 / topic.votes }
  end
end

class Vote < ActiveRecord::Base
  belongs_to :topic
end