title: Item does not decrease Django
body: <p>Hello I want to make a feature, that if I click quantity decrease by 1 but it does not decrease and only returns HttpResponse.</p>
  <pre><code>class CartView(TemplateView):
      template_name = &quot;shop/cart.html&quot;
      def get_context_data(self, **kwargs):
          context = super().get_context_data(**kwargs)
          context['cart'] = Cart.objects.annotate(
          price=Sum(F('orderitem__item__price') * F('orderitem__quantity'))
          ).get(order_user= self.request.user)
          cart = context['cart']
          cart.total = cart.price
          cart.save()
          context['order_items'] = OrderItem.objects.filter(cart=cart)
          return context
      def post(self, request):
          if 'minus' in request.POST:
              cart = Cart.objects.get(order_user=self.request.user)
              primary = request.POST.get('orderitem__id')
              item = OrderItem.objects.filter(id=primary, cart=cart)
              item.quantity = F('quantity') - 1
              item.update()
              return HttpResponse(&quot;cart uptaded&quot;)
  </code></pre>