how can i index the sum of tow attributes with thinking sphinx ?

i have option model with this structure:

Class Option < ActiveRecord::Base

has_one :base_price, :class_name => "OptionPrice", :conditions =>
{:base => true}

  define_index do
    indexes base_price.provider_price, :as => :p_base_price
    indexes base_price.margin_price, :as => :m_base_price
    indexes "SUM(p_base_price, m_base_price)", :as => :base_price

and in the controller i want to do :

@products =[:search], :conditions =>{:base_price
=> 15 } ,
        :page => params[:page], :per_page => 2)
this is a wrong solution of course but i think any one can understand
what i want to do : searching products having base_price = 15
(base_price = provider_price + margin_price, which are OptionPrice
How can i solve this issue , i'm using thinking_sphinx 2.0.3 and RAILS
Best Regards