title: yii2 gridview filter a count/sum on a second table column
body: <p>I have these related tables: <code>order</code>, <code>order_product (order_id, product_id, quantity, product_price)</code> and <code>product</code>.
  I am using <code>yii2</code> grid view to display the order model with it's total amount that can be calculated in sql like 
  <code>select SUM(p.quantity*p.product_price)</code> as <code>total from order_product p GROUP by order_id</code>
  or with a php getter and the number of products ordered that I can get easily with a hasmany. 
  My problem is the gridview filter. 
  How can I set the search and sort of these columns in the gridview?</p>