Aggregate Sum for Grid by Association

Ok, I'm new at this , have a grid of order items on an order page.  I have a total field for the order items (qty * price).  Want a total for the whole grid, the aggregate feature says "Aggregation is only supported in grids with an xpath source."   There has to be a simple way to show the total for the grid, what's the easiest way?
1 answers

Make sure the source for your grid is set to xpath in stead of the default database option.