Implement non-destructive order updates

uncloud_pay/views.py

@@ -13,6 +13,7 @@ from hardcopy import bytestring_to_pdf
 from django.core.files.temp import NamedTemporaryFile
 from django.http import FileResponse
 from django.template.loader import render_to_string
+from copy import deepcopy
 
 import json
 import logging
@@ -304,6 +305,7 @@ class AdminBillViewSet(BillViewSet):
 class AdminOrderViewSet(mixins.ListModelMixin,
         mixins.RetrieveModelMixin,
         mixins.CreateModelMixin,
+        mixins.UpdateModelMixin,
         viewsets.GenericViewSet):
     serializer_class = OrderSerializer
     permission_classes = [permissions.IsAdminUser]
@@ -314,6 +316,32 @@ class AdminOrderViewSet(mixins.ListModelMixin,
     def get_queryset(self):
         return Order.objects.all()
 
+    # Updates create a new order and terminate the 'old' one.
+    @transaction.atomic
+    def update(self, request, *args, **kwargs):
+       order = self.get_object()
+       partial = kwargs.pop('partial', False)
+       serializer = self.get_serializer(order, data=request.data, partial=partial)
+       serializer.is_valid(raise_exception=True)
+
+       # Clone existing order for replacement.
+       replacing_order = deepcopy(order)
+
+       # Yes, that's how you make a new entry in DB:
+       #   https://docs.djangoproject.com/en/3.0/topics/db/queries/#copying-model-instances
+       replacing_order.pk = None
+
+       for attr, value in serializer.validated_data.items():
+          setattr(replacing_order, attr, value)
+
+       # Save replacing order and terminate 'previous' one.
+       replacing_order.save()
+       order.replaced_by = replacing_order
+       order.save()
+       order.terminate()
+
+       return Response(replacing_order)
+
     @action(detail=True, methods=['post'])
     def terminate(self, request, pk):
         order = self.get_object()