openerp - Edit compute_amount -

i had edit funtion of compute_amount okay problem when pay invoice i'm still getting old amount before change has work on this??? new compute_amount

@api.one @api.depends('invoice_line.price_subtotal', 'tax_line.amount','timbre','partner_id') def _compute_amount(self):     if self.type == 'out_invoice':         if self.partner_id.rs == true:             print'rs',self.partner_id.rs             self.amount_untaxed = sum(line.price_subtotal line in self.invoice_line)             self.amount_tax = sum(line.amount line in self.tax_line)             self.amount_rs = (self.amount_untaxed + self.amount_tax) * 0.01             self.amount_total =  self.amount_untaxed + self.amount_tax + self.amount_rs + self.timbre         else :             self.amount_untaxed = sum(line.price_subtotal line in self.invoice_line)             self.amount_tax = sum(line.amount line in self.tax_line)             self.amount_total =  self.amount_untaxed + self.amount_tax + self.timbre      else :         self.amount_untaxed = sum(line.price_subtotal line in self.invoice_line)         self.amount_tax = sum(line.amount line in self.tax_line)         self.amount_total = self.amount_untaxed + self.amount_tax 

if on_change method , field has readonly attribute, write method won't new value.
problem solution may if create invisible mirror function field.