Change semantics of changing orders (#1260)

* Change semantics of changing orders

This basically does two things to the "Change products" view of orders and the
OrderChangeManager program API:

1) It decouples changing items or subevents from changing prices.
   OrderChangeManager.change_item() and .change_subevent() no longer
   touch the price of a position. Instead .change_price() needs to be
   called explicitly. However, a client-side JavaScript component now
   *proposes* a new price based on the changed item or subevent.

2) The user interface now exposes the possibility of doing multiple
   things at the same time, i.e. changing the item, subevent and price
   in the same operation. OrderChangeManager already allowed this
   before.

(1) is basically a consequence of (2), while (2) is a prerequesite for
e.g. the `seating` branch, where changing the subevent will always
require changing the seat.

* Add tests for price calculation API

src/pretix/api/views/order.py

@@ -24,11 +24,12 @@ from pretix.api.models import OAuthAccessToken
 from pretix.api.serializers.order import (
     InvoiceSerializer, OrderCreateSerializer, OrderPaymentSerializer,
     OrderPositionSerializer, OrderRefundCreateSerializer,
-    OrderRefundSerializer, OrderSerializer,
+    OrderRefundSerializer, OrderSerializer, PriceCalcSerializer,
 )
+from pretix.base.i18n import language
 from pretix.base.models import (
-    CachedCombinedTicket, CachedTicket, Device, Event, Invoice, Order,
-    OrderPayment, OrderPosition, OrderRefund, Quota, TeamAPIToken,
+    CachedCombinedTicket, CachedTicket, Device, Event, Invoice, InvoiceAddress,
+    Order, OrderPayment, OrderPosition, OrderRefund, Quota, TeamAPIToken,
     generate_position_secret, generate_secret,
 )
 from pretix.base.payment import PaymentException
@@ -42,10 +43,12 @@ from pretix.base.services.orders import (
     OrderChangeManager, OrderError, approve_order, cancel_order, deny_order,
     extend_order, mark_order_expired, mark_order_refunded,
 )
+from pretix.base.services.pricing import get_price
 from pretix.base.services.tickets import generate
 from pretix.base.signals import (
     order_modified, order_placed, register_ticket_outputs,
 )
+from pretix.base.templatetags.money import money_filter
 
 
 class OrderFilter(FilterSet):
@@ -622,6 +625,83 @@ class OrderPositionViewSet(mixins.DestroyModelMixin, viewsets.ReadOnlyModelViewS
                 return prov
         raise NotFound('Unknown output provider.')
 
+    @action(detail=True, methods=['POST'], url_name='price_calc')
+    def price_calc(self, request, *args, **kwargs):
+        """
+        This calculates the price assuming a change of product or subevent. This endpoint
+        is deliberately not documented and considered a private API, only to be used by
+        pretix' web interface.
+
+        Sample input:
+
+        {
+            "item": 2,
+            "variation": null,
+            "subevent": 3
+        }
+
+        Sample output:
+
+        {
+            "gross": "2.34",
+            "gross_formatted": "2,34",
+            "net": "2.34",
+            "tax": "0.00",
+            "rate": "0.00",
+            "name": "VAT"
+        }
+        """
+        serializer = PriceCalcSerializer(data=request.data, event=request.event)
+        serializer.is_valid(raise_exception=True)
+        data = serializer.validated_data
+        pos = self.get_object()
+
+        try:
+            ia = pos.order.invoice_address
+        except InvoiceAddress.DoesNotExist:
+            ia = InvoiceAddress()
+
+        kwargs = {
+            'item': pos.item,
+            'variation': pos.variation,
+            'voucher': pos.voucher,
+            'subevent': pos.subevent,
+            'addon_to': pos.addon_to,
+            'invoice_address': ia,
+        }
+
+        if data.get('item'):
+            item = data.get('item')
+            kwargs['item'] = item
+
+            if item.has_variations:
+                variation = data.get('variation') or pos.variation
+                if not variation:
+                    raise ValidationError('No variation given')
+                if variation.item != item:
+                    raise ValidationError('Variation does not belong to item')
+                kwargs['variation'] = variation
+            else:
+                variation = None
+                kwargs['variation'] = None
+
+            if pos.voucher and not pos.voucher.applies_to(item, variation):
+                kwargs['voucher'] = None
+
+        if data.get('subevent'):
+            kwargs['subevent'] = data.get('subevent')
+
+        price = get_price(**kwargs)
+        with language(data.get('locale') or self.request.event.settings.locale):
+            return Response({
+                'gross': price.gross,
+                'gross_formatted': money_filter(price.gross, self.request.event.currency, hide_currency=True),
+                'net': price.net,
+                'rate': price.rate,
+                'name': str(price.name),
+                'tax': price.tax,
+            })
+
     @action(detail=True, url_name='download', url_path='download/(?P<output>[^/]+)')
     def download(self, request, output, **kwargs):
         provider = self._get_output_provider(output)