Hi all,

I've created a field Transport charges in sale order which fetches the amount from delivery method field in sale order. This field is used in the computation of margin. 

Margin now is "Selling price - Cost price". but I've altered it as "Selling price - Cost price - Transport Charges" and it is working fine when i create the order freshly but if I edit the transport charges amount in a already created order the margin is not getting updated, It remains the old margin. The margin is not getting computed and updated again.

Can anyone help me how to update or where am I going wrong

The code to compute is

_inherit = "sale.order"
    def _product_margin(self, cr, uid, ids, field_name, arg, context=None):
        result = {}
        for sale in self.browse(cr, uid, ids, context=context):
            result[sale.id] = 0.0
            for line in sale.order_line:
                result[sale.id] += line.margin or 0.0
            result[sale.id] = result[sale.id] - sale.transport
        return result

_columns = {
        'transport':fields.float('transport charges', digits=(16,2)),            
    }