Is it possible to implement field value calculation directly in the model?

Question

Django

Citizens experts, there was such question. I have a need to create a model in which the values ​​of some fields will be calculated based on the values ​​of other fields. Here's how I implemented it (the example is simplified, but conveys the essence):

class Zzz(Superclass):
    aaa = models.ForeignKey(Aaa)
    bbb = models.ForeignKey(Bbb)
    ccc = models.ForeignKey(Ccc)

    @property
    def eee(self):
        return self.aaa, self.bbb, self.ccc

    @property
    def ddd(self):
        return reduce((lambda x, y: x + y), [e.some_param for e in self.eee])

Do I understand correctly that the value of ddd will be recalculated with each new access? Is it possible to remake the model code so that the field value is calculated once when creating a record, and then simply read from the database? Or is the only option - views?

Answer 1

It is possible to alter something, but then this field will have to be added to the scheme. And fill on save:

class Zzz(Superclass):
    aaa = models.ForeignKey(Aaa)
    bbb = models.ForeignKey(Bbb)
    ccc = models.ForeignKey(Ccc)
    # Объявили поле и на всякий случай запретили редактировать его из админки или через форму.
    ddd = models.IntegerField(editable=False, default=0) 

    @property
    def eee(self):
        return self.aaa, self.bbb, self.ccc

    def save(self, *args, **kwargs):
        # Непосредственно перед физическим сохранением вычисляем значение поля
        self.ddd = reduce((lambda x, y: x + y), [e.some_param for e in self.eee])
        return super(Zzz, self).save(*args, **kwargs)