My Proposal model is defined as follows:
class Proposal(models.Model):
scheduled_time = models.DateTimeField()
duration = models.IntegerField() # sto
F()
-expressions works only with django (as they return instances of specific objects). You can't pass them to timedelta or whatever except some of the django queryset methods.
So you'd better add new field to you model:
class Proposal(models.Model):
scheduled_time = models.DateTimeField()
duration = IntegerField()
end = models.DateTimeField(default=self.scheduled_time + timedelta(minutes=self.duration))