class DairyManager(models.Manager):
def get_queryset(self):
return super(DairyManager, self).get_queryset().extra(select={'totalsum':'count*cost'})
class Dairy(models.Model):
...
count = models.IntegerField("Количество")
cost = models.FloatField('Цена')
...
total = DairyManager()
objects = models.Manager()
qs1 = Dairy.total.all()
qs2 = Dairy.objects.all()