如何通过Propel获取请求中的元素总和？

I'm trying to get the sum of variables contained in my object, and I don't know how to do it. I know it's not very clear, so here is an example.

I'm doing a request to obtain all discounts from one User. I now want to get the sum of all the discounts he had. In SQL, it would be:

SELECT SUM(c.discount_price) AS SumDiscounts FROM my_table c GROUP BY c.user_id

How can I do it in propel?

Try this query:

    $collection = MyTableQuery::create()
        ->withColumn('SUM(c.discount_price)', 'SumDiscounts')
        ->groupByUserId()
        ->find();