Use OrderProductRelationForm to validate stock when adding a new product to order, also make it possible to update the quantity of a product on the product detail page if it is already in the current order.

src/profiles/templates/shop/order_detail.html

@@ -48,7 +48,7 @@
             <td>
               {{ form.instance.total|currency }}
             <td>
-              {% bootstrap_button '<i class="glyphicon glyphicon-remove"></i>' button_type="submit" button_class="btn-danger" name="remove_product" value=order_product.pk %}
+              {% bootstrap_button '<i class="glyphicon glyphicon-remove"></i>' button_type="submit" button_class="btn-danger" name="remove_product" value=form.instance.pk %}
 
         {% endfor %}
 

src/shop/forms.py

@@ -4,11 +4,8 @@ from django.forms import modelformset_factory
 from shop.models import OrderProductRelation
 
 
-class AddToOrderForm(forms.Form):
-    quantity = forms.IntegerField(initial=1)
-
-
 class OrderProductRelationForm(forms.ModelForm):
+
     class Meta:
         model = OrderProductRelation
         fields = ['quantity']

src/shop/templates/product_detail.html

@@ -40,7 +40,9 @@
 
     {% if product.is_stock_available %}
 
-    <h3>Add to order</h3>
+    <h3>{% if already_in_order %}Update order{% else %}Add to order{% endif %}</h3>
+
+    {% if already_in_order %}<p>You already have this product in your order.</p>{% endif %}
 
     {% if user.is_authenticated %}
 
@@ -49,7 +51,9 @@
     <form method="POST">
       {% csrf_token %}
       {% bootstrap_form form %}
-      {% bootstrap_button "Add to order" button_type="submit" button_class="btn-primary" %}
+        <button type="submit" class="btn btn-primary">
+          {% if already_in_order %}Update{% else %}Add{% endif %}
+        </button>
     </form>
 
     {% else %}

src/shop/templates/shop_index.html

@@ -59,16 +59,14 @@ Shop | {{ block.super }}
 
             {% if product.stock_amount %}
 
-            {% if product.left_in_stock <= 10 %}
-            <div class="label label-info">
-              Only {{ product.left_in_stock }} left!
-            </div>
-            {% endif %}
-
             {% if product.left_in_stock == 0 %}
             <div class="label label-danger">
               Sold out!
             </div>
+            {% elif product.left_in_stock <= 10 %}
+            <div class="label label-info">
+              Only {{ product.left_in_stock }} left!
+            </div>
             {% endif %}
 
             {% endif %}

src/shop/views.py

@@ -41,7 +41,7 @@ from .coinify import (
     process_coinify_invoice_json
 )
 from .epay import calculate_epay_hash, validate_epay_callback
-from .forms import AddToOrderForm, OrderProductRelationFormSet
+from .forms import OrderProductRelationFormSet, OrderProductRelationForm
 
 logger = logging.getLogger("bornhack.%s" % __name__)
 
@@ -219,11 +219,39 @@ class ShopIndexView(ListView):
 class ProductDetailView(FormView, DetailView):
     model = Product
     template_name = 'product_detail.html'
-    form_class = AddToOrderForm
+    form_class = OrderProductRelationForm
     context_object_name = 'product'
 
+    def get_form_kwargs(self):
+        kwargs = super().get_form_kwargs()
+        kwargs['instance'] = self.opr
+        return kwargs
+
+    def get_initial(self):
+        return {'quantity': self.opr.quantity}
+
+    def get_context_data(self, **kwargs):
+        if hasattr(self.opr, 'order'):
+            kwargs['already_in_order'] = True
+
+        return super().get_context_data(**kwargs)
+
     def dispatch(self, request, *args, **kwargs):
-        if not self.get_object().category.public:
+        self.object = self.get_object()
+
+        try:
+            self.opr = OrderProductRelation.objects.get(
+                order__user=self.request.user,
+                order__open__isnull=False,
+                product=self.object
+            )
+        except OrderProductRelation.DoesNotExist:
+            self.opr = OrderProductRelation(
+                product=self.get_object(),
+                quantity=1,
+            )
+
+        if not self.object.category.public:
             # this product is not publicly available
             raise Http404("Product not found")
 
@@ -235,31 +263,13 @@ class ProductDetailView(FormView, DetailView):
         product = self.get_object()
         quantity = form.cleaned_data.get('quantity')
 
-        # do we have an open order?
+        if not hasattr(self.opr, 'order'):
-        try:
+            self.opr.order = Order.objects.create(
-            order = Order.objects.get(
-                user=self.request.user,
-                open__isnull=False
-            )
-        except Order.DoesNotExist:
-            # no open order - open a new one
-            order = Order.objects.create(
                 user=self.request.user,
             )
 
-        # get product from kwargs
+        self.opr.quantity = quantity
-        if product in order.products.all():
+        self.opr.save()
-            # this product is already added to this order,
-            # increase count by quantity
-            OrderProductRelation.objects.filter(
-                product=product,
-                order=order
-            ).update(quantity=F('quantity') + quantity)
-        else:
-            order.orderproductrelation_set.create(
-                product=product,
-                quantity=quantity,
-            )
 
         messages.info(
             self.request,