There will be product of different uom's in PO line,, how can we combine that?? eg: 6 gm and 5 liter and 7 No.

If you still want to calculate irrespective of that,, you can use the function

first declare a function and  functional field:-

    def _compute_total(self, cr, uid, ids, name, args, context=None):
        res = {}
        for invoice in self.browse(cr, uid, ids, context=context):
            res[invoice.id] = {
                'amount_total': 0.0,
            }
            for line in invoice.order_line:
                res[invoice.id]['amount_total'] += line.product_qty
        return res

'amount_total': fields.function(_compute_total, string='Total Quantity')

If you want to cosider different Units you can update function like this:-

res[invoice.id]['amount_total'] += line.product_qty / line.product_uom.factor

Hey! Can anyone help me how I can add this function using the dev tools?

I want to add all the quantities irrespective of their UoM's. 

Hi Willie, the sum of quantity may provide wrong information because the quantity depends on the UoM and the product.  For example, what is 1 ltr of milk + 1 jug of milk?  Or what is 1 dozen of apple + 1 dozen of orange?

Technically, however you get the sum of the figures, by adding the  sum="XXXX" in the view's field definition, e.g. <field name="amount_total" sum="Total Amount"/>.