Calculating a date inside the Model Django timedelta

Question

I am declaring a function (data_prox_prog) to calculate a future date and store in the model, but it is not happening as expected. Follows code:

class ProgressaoDocente(models.Model):
servidor = models.ForeignKey(Servidor, on_delete=models.PROTECT)
tipo_progressao = models.CharField(choices=const.TIPO_PROGRESSAO_DOC, max_length=1)
classe = models.CharField(choices=const.CLASSE_DOCENTE, max_length=1)
nivel = models.CharField(choices=const.NIVEL_DOCENTE, max_length=1)
data_progressao = models.DateField()
portaria = models.OneToOneField(Portaria, on_delete=models.CASCADE)
data_prox_progressao = models.DateField(blank=True, null=True)

class Meta:
    verbose_name_plural = 'Progressão Docente'

def data_prox_prog(self):
    self.data_prox_prog = (self.data_progressao + timedelta(year=2))
    self.save()

def __str__(self):
    return "{} {}, {}/{}".format(self.servidor, self.tipo_progressao, self.classe, self.nivel)

Answer 1

I was able to solve it using pre-save Django Signals. What is expected is that when a new 'Teacher Progression', from data_progression, is set the value of 'data_prox_progression', counting 2 more years of 'data_progression', follows solution:

class ProgressaoDocente(models.Model):
    servidor = models.ForeignKey(Servidor, on_delete=models.PROTECT)
    tipo_progressao = models.CharField(choices=const.TIPO_PROGRESSAO_DOC, 
    max_length=1)
    classe = models.CharField(choices=const.CLASSE_DOCENTE, max_length=1)
    nivel = models.CharField(choices=const.NIVEL_DOCENTE, max_length=1)
    data_progressao = models.DateField()
    portaria = models.OneToOneField(Portaria, on_delete=models.CASCADE)
    data_prox_progressao = models.DateField(blank=True, null=True)


    class Meta:
        verbose_name_plural = 'Progressão Docente'

    def __str__(self):
        return "{} {}, {}/{}".format(self.servidor, self.tipo_progressao, 
        self.classe, self.nivel)

@receiver(pre_save, sender=ProgressaoDocente)
def callback_progressao_docente(sender, instance, *args, **kwargs):
    instance.data_prox_progressao = (instance.data_progressao + 
    relativedelta(years=+2))