Changing the format of a field depending on another fiel in the same model

Question

Using Django I'm trying to change the format of a field depending on another field in the same model.

I have a group of persons who give me a detail which could be in two different formats: time, or decimal. I would like to have all the results in the same table. Is that possible?

This is what I tried in models.py:

class Time_manager(models.Manager):

def get_queryset(self):
    return super().get_queryset().filter(result_format='T')

class Kilo_manager(models.Manager):
    def get_queryset(self):
    return super().get_queryset().filter(result_format='K')


class Guest(models.Model):
   name = models.CharField(max_length=30)
   result_format=models.CharField(max_length=5, default="K")
   result= models.DecimalField(max_digits=6,decimal_places=2)

class Guest_time(Guest):

   objects=Time_manager()

   class Meta:
    proxy = True

def formato_resultado(self):
    self.resultado=models.DurationField()

But when I try to add a new Guest_time I'm not able to do it. I get an error asking me to introduce a decimal data.

Is there a way of doing this using the same table?