1
One solution is
class Bid(models.Model):
product = models.ForeignKey(Auction, related_name='auction')
current_amount = models.PositiveIntegerField()
def save(self, *args, **kwargs):
if not self.current_amount:
self.current_amount = self.product.minimum_price
super(Bid, self).save(*args, **kwargs)
Source:stackexchange.com