Datagrid custom column

This is not a Radzen datagrid specific problem but one may have a solution that I could not see.
I have two tables, one is the parent of the other. I need to show the parent table in a datagrid with the addition of a field that is summed from a field from the child table.
Is there any suggestion for this problem?

Here is an example using this demo:

<RadzenDataGridColumn Title="Total Quantity">
    <Template>
        @(context.OrderDetails.Sum(od => od.Quantity))
    </Template>
</RadzenDataGridColumn>