sql query in a more "rails 3 way"

Hi people

I want to know if you know how to write the following sql query in a more "rails way"

UPDATE buy_order_detail SET saldo = saldo - detail_purchase.cantidad FROM purchase_detail            INNER JOIN purchase ON purchase.id = purchaser_detail.purchase_id            INNER JOIN waybill ON waybill.id = purchase.waybill_id            INNER JOIN buy_order ON buy_order.id = waybill.order_id            INNER JOIN buy_order_detail ON buy_order.id = buy_order_detail.order_id WHERE purchase_detail.product_id = buy_order_detail.product_id