Hello azlam

Hope you are doing well.

For 'Purchase Discount for All Items'.

- I put one custom field 'discount' integer.

- So, Whenever we will add any amount in purchase order's 'discount' fields, It will update amount_untaxed, amount_tax and amount_total accordingly.

Code : 

class PurchaseOrder(models.Model):

    _inherit = 'purchase.order'

    discount_total = fields.Float(string='Discount Value')

    @api.depends('discount_value', 'discount_type', 'discount_total', 'order_line.price_total')

    def _amount_all(self):

        super(PurchaseOrder, self)._amount_all()

        for order in self:

            discount_total = 0.0

            order.update({

                'discount_total': discount_total,

                'amount_untaxed': order.amount_untaxed - discount_total,

                'amount_tax': ((order.amount_untaxed - discount_total) * order.order_line.taxes_id.amount / 100),

                'amount_total' : (order.amount_untaxed - discount_total) + ((order.amount_untaxed - discount_total) * order.order_line.taxes_id.amount / 100),

            })  

Also, We can also change calculations according to our need.

Please reach out to us if any queries.

I Hope this information proves helpful to you.

Thanks & Regards,

Email:   odoo@aktivsoftware.com           

Skype: kalpeshmaheshwari 

Thank you for your answer and solution.