How to save a result of multiplication

Hi everyone i build a web application in Django 3.1.7 and i try to do the next opeation(multiplication).

in my models a have:

class Articles(models.Model):

     quantity = models.PositiveIntegerField()

    cost_buy = models.DecimalField(max_digits=10, decimal_places=2)

    total_todo = models.DecimalField(max_digits=10, decimal_places=3)

@property

def total_valores(self):

     return (self.cost_buy*self.quantity)

def saveit(self):

    self.total_todo = self.total_valores

    super (Articles, self).save()

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so, I was tried with this functions , but it's doesn't save anything

would you give me some recomendations.

Thank you very much in advance and I remain at your service regards

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