思考Sphinx类型与has_many关联的整数

Question

示例：我们有一个模型Book（标题：string desc：text）和一个模型Reader（some_integer：integer book_id：integer name：string）

class Book < ActiveRecord::Base


has_many :readers

  define_index do    
    indexes id
    indexes title      
    indexes descr
    join    readers   
    has     readers(:wert_integer), :as => :wert_integer, :sortable => true, :type => :integer
    has     readers(:first_name), :as => :first_name, :sortable => true
    has     readers(:last_name), :as => :last_name, :sortable => true
    set_property :delta => :delayed
  end
end

class Reader < ActiveRecord::Base
  belongs_to :book
end

在尝试保存图书时，我们遇到了此错误：

undefined method `wert_integer' for []:ActiveRecord::Relation

我们做错了什么？