Ruby on Rails范围计算数据

Question

我有一个User和Shop模型。关系是：User has_many :shops。

User模型中有一个范围：

scope :order_by, ->(order_by) { order("#{order_by} DESC") }

如何按shops计数对用户进行排序？

像

<%= select_tag(:order_by, options_for_select([["Name", "name"], ["E-mail", "email"], 
                                                ["Shops count", "user.shops.count"], 
                                                ["Date of creation", "created_at"]],
                                                 "Date of creation")) %>