Django: How to set a field's default value to the value of a field in a parent model -

say have following model

class sammich(models.model):     name = models.charfield(max_length=200)     ratio_of_cheese_to_meat = models.floatfield(default=0.3) 

i'd able create new model has field pulls default ratio_of_cheese_to_meat of sammich class

class delisammich(models.model):     sammich = models.foriegnkey(sammich)     type_of_meat = models.charfield(max_length=200)     type_of_cheese = models.charfield(max_length=200)     ratio_of_cheese_to_meat = models.floatfield(default=sammich.objects.get(pk=sammich.id).ratio_of_cheese_to_meat) 

which isn't working.

one option override model's save() method , default:

class delisammich(models.model):     sammich = models.foreignkey(sammich)     type_of_meat = models.charfield(max_length=200)     type_of_cheese = models.charfield(max_length=200)     ratio_of_cheese_to_meat = models.floatfield()      def save(self, *args, **kwargs):         if not self.ratio_of_cheese_to_meat:             self.ratio_of_cheese_to_meat = self.sammich.ratio_of_cheese_to_meat         super(delisammich, self).save(*args, **kwargs)