title: Can't getting total when a product has discount price
body: <p>I am trying to get total amount. Like when a product has discount price the total will be discount_price * quantity = total</p>
  <p>For that i have this model and view:</p>
  <pre><code>class Order(models.Model):
      product = models.ForeignKey(Product, on_delete=models.CASCADE, related_name=&quot;product&quot;)
      customer = models.ForeignKey(Customer, on_delete=models.CASCADE)
      quantity = models.IntegerField(default=1)
      fname = models.CharField(max_length=100, null=True)
      address = models.CharField(max_length=1000, null=True)
      phone = models.CharField(max_length=12, null=True)
      price = models.FloatField()
      discount_price = models.FloatField(null=True, blank=True)
      date = models.DateField(datetime.datetime.today, null=True)
      status = models.CharField(choices=STATUS_S, blank=True, null=True, max_length=255)
      payment_method = models.ForeignKey(PaymentMethod, on_delete=models.CASCADE, blank=True, null=True)
      total = models.FloatField(null=True, blank=True)
  
      def save(self, *args, **kwargs):
          if self.discount_price:
              self.total = self.discount_price * self.quantity
          else:
              self.total = self.price * self.quantity
          return super().save(self, *args, **kwargs)
  
  
  class Checkout(View):
      def post(self, request):
          fname = request.POST.get(&quot;fname&quot;)
          phone = request.POST.get(&quot;phone&quot;)
          address = request.POST.get(&quot;address&quot;)
          cart = request.session.get(&quot;cart&quot;)
          customer = request.session.get(&quot;customer&quot;)
          products = Product.get_products_id(list(cart.keys()))
  
          for product in products:
              if discount_price:
                  order = Order(
                      customer=Customer(id=customer[&quot;id&quot;]),
                      product=product,
                      fname=fname,
                      discount_price=product.discount_price,
                      phone=phone,
                      address=address,
                      quantity=cart.get(str(product.id)),
                  )
              else:
                  order = Order(
                      customer=Customer(id=customer[&quot;id&quot;]),
                      product=product,
                      fname=fname,
                      price=product.price,
                      phone=phone,
                      address=address,
                      quantity=cart.get(str(product.id)),
                  )
  
              order.save()
  
          request.session[&quot;cart&quot;] = {}
  
          return redirect(&quot;cart&quot;)
  </code></pre>