Ruby on rails 按反向关联上的关联属性计数排序

关联排序问题：）

我正试图找出如何通过反向关联对一个有很多挑战的用户进行排序，该反向关联基于发送和接收的挑战总数

以下是协会代码：

class User < ActiveRecord::Model

  has_many :sent_challenges, class_name: "Challenge", foreign_key: :challenger_id, inverse_of: :challenger
  has_many :received_challenges, class_name: "Challenge", foreign_key: :opponent_id, inverse_of: :opponent


  class << self
    def in_order
      # return users sorted by amount of sent + received challenges
      order("…")
    end
  end

end

class用户