# On\_change is taking the old field value
**URL:** https://discuss.tryton.org/t/on-change-is-taking-the-old-field-value/3069
**Category:** Developer
**Tags:** accounting
**Created:** [July 25, 2020, 3:33am UTC](https://discuss.tryton.org/t/on-change-is-taking-the-old-field-value/3069 "2020-07-25T03:33:50Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@German](https://discuss.tryton.org/u/German)
#### Post date: [July 25, 2020, 3:33am UTC](https://discuss.tryton.org/t/on-change-is-taking-the-old-field-value/3069/1 "2020-07-25T03:33:50Z")
Dear Tryton team.
I’m working on a discount functionally for invoice:
1. User has to input the % of discount
2. Tryton will calculate the amount of this particular discount ( from payable amount and I need this field on db , so I can’t use function field)
3. I’ll show a total-discount (using a function field)
Everything seems good, but I’m doing something wrong with the on\_change method, when I put 10% and then let’s say I change for 20% the invoice\_discount\_amount field is calculated with 10%, like calculated with previous value of % in all cases.
Any help will be appreciated:
```
class Invoice(metaclass=PoolMeta):
__name__ = 'account.invoice'
invoice_discount = fields.Numeric('Descuento %')
invoice_discount_amount= fields.Numeric('Monto del descuento')
invoice_with_discount = fields.Function(fields.Numeric('Total con descuento',depends=['invoice_discount_amount'
,'total_amount']),'amount_with_discount')
def amount_with_discount(self, name):
try:
amount_with_discount = self.total_amount - self.invoice_discount_amount
except:
amount_with_discount = self.total_amount
return amount_with_discount
@fields.depends('untaxed_amount','tax_amount','invoice_discount')
def on_change_invoice_discount(self, name=None):
if self.untaxed_amount:
amount = self.untaxed_amount + self.tax_amount
discount_amount = Decimal(self.invoice_discount/100) * amount
self.invoice_discount_amount = discount_amount
```
---
### Author:  [@German](https://discuss.tryton.org/u/German)
#### Post date: [July 25, 2020, 3:24pm UTC](https://discuss.tryton.org/t/on-change-is-taking-the-old-field-value/3069/2 "2020-07-25T15:24:42Z")
Well, It’s working using the **on\_change\_with** on destination field, here’s the working method in case someone has a similar issue 😉 :
```
@fields.depends('untaxed_amount','tax_amount','invoice_discount')
def on_change_with_invoice_discount_amount(self, name=None):
if self.untaxed_amount:
amount = self.untaxed_amount + self.tax_amount
discount_amount = Decimal(self.invoice_discount/100) * amount
return discount_amount
else:
return 0
```
---
### Author:  [@system](https://discuss.tryton.org/u/system)
#### Post date: [August 24, 2020, 3:24pm UTC](https://discuss.tryton.org/t/on-change-is-taking-the-old-field-value/3069/3 "2020-08-24T15:24:45Z")
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