Orders: Fix N+1 query issues on order detail and change page

There is one I couldn't fix: Loading the list of tax rules for every
select box on the OrderChange page. Unfortunately, Django has a
cache-breaking .all() in ModelChoiceField and that would need nasty
patching that didn't feel worth it

src/pretix/control/forms/orders.py

@@ -562,9 +562,7 @@ class OrderPositionChangeForm(forms.Form):
         if instance.addon_to_id:
             del self.fields['operation_split']
 
-        if not instance.seat and not (
-                instance.item.seat_category_mappings.filter(subevent=instance.subevent).exists()
-        ):
+        if not instance.seat and not instance._seat_allowed:
             del self.fields['seat']
 
         choices = [

src/pretix/control/templates/pretixcontrol/order/index.html

@@ -833,7 +833,7 @@
                                 <strong>{% trans "Pending total" %}</strong>
                             </div>
                             <div class="col-md-3 col-xs-6 col-md-offset-5 price">
-                                <strong>{{ order.pending_sum|money:event.currency }}</strong>
+                                <strong>{{ pending_sum|money:event.currency }}</strong>
                             </div>
                             <div class="clearfix"></div>
                         </div>

src/pretix/control/views/orders.py

@@ -81,7 +81,7 @@ from pretix.base.i18n import language
 from pretix.base.models import (
     CachedFile, CachedTicket, Checkin, Invoice, InvoiceAddress, Item,
     ItemVariation, LogEntry, Order, QuestionAnswer, Quota,
-    ScheduledEventExport, generate_secret,
+    ScheduledEventExport, generate_secret, SeatCategoryMapping,
 )
 from pretix.base.models.orders import (
     CancellationRequest, OrderFee, OrderPayment, OrderPosition, OrderRefund,
@@ -564,10 +564,11 @@ class OrderDetail(OrderView):
         })
         ctx['display_locale'] = dict(settings.LANGUAGES)[self.object.locale or self.request.event.settings.locale]
 
-        ctx['overpaid'] = self.order.pending_sum * -1
+        pending_sum = self.order.pending_sum
+        ctx['overpaid'] = pending_sum * -1
         ctx['download_buttons'] = self.download_buttons
         ctx['payment_refund_sum'] = self.order.payment_refund_sum
-        ctx['pending_sum'] = self.order.pending_sum
+        ctx['pending_sum'] = pending_sum
         ctx['uncancelled_invoice'] = self.order.invoices.exclude(
             Exists(self.order.invoices.filter(refers=OuterRef('pk'), is_cancellation=True))
         ).exclude(is_cancellation=True).first()
@@ -603,6 +604,7 @@ class OrderDetail(OrderView):
             Prefetch('answers', queryset=QuestionAnswer.objects.prefetch_related('options').select_related('question')),
             Prefetch('all_checkins', queryset=Checkin.all.select_related('list').order_by('datetime')),
             Prefetch('print_logs', queryset=PrintLog.objects.select_related('device').order_by('datetime')),
+            Prefetch('subevent', queryset=self.request.event.subevents.all()),
         ).order_by('positionid')
 
         positions = []
@@ -1981,24 +1983,46 @@ class OrderChange(OrderView):
     def items(self):
         return self.request.event.items.prefetch_related('variations', 'tax_rule').all()
 
+    @cached_property
+    def tax_rules(self):
+        return self.request.event.tax_rules.all()
+
     @cached_property
     def fees(self):
         fees = list(self.order.fees.all())
         for f in fees:
-            f.form = OrderFeeChangeForm(prefix='of-{}'.format(f.pk), instance=f,
-                                        data=self.request.POST if self.request.method == "POST" else None)
+            f.form = OrderFeeChangeForm(
+                prefix='of-{}'.format(f.pk),
+                instance=f,
+                tax_rules=self.tax_rules,
+                data=self.request.POST if self.request.method == "POST" else None
+            )
         return fees
 
     @cached_property
     def positions(self):
         positions = list(self.order.positions.select_related(
             'item', 'item__tax_rule', 'used_membership', 'used_membership__membership_type', 'tax_rule',
-            'seat', 'subevent',
-        ).prefetch_related('granted_memberships'))
+            'seat',
+        ).prefetch_related(
+            Prefetch(
+                'subevent',
+                queryset=self.request.event.subevents.all(),
+            ),
+            'granted_memberships',
+            'issued_gift_cards',
+            'addons',
+        ).annotate(
+            _seat_allowed=Exists(SeatCategoryMapping.objects.filter(subevent=OuterRef("subevent"), product=OuterRef("item")))
+        ))
         for p in positions:
-            p.form = OrderPositionChangeForm(prefix='op-{}'.format(p.pk), instance=p, items=self.items,
-                                             initial={'seat': p.seat.seat_guid if p.seat else None},
-                                             data=self.request.POST if self.request.method == "POST" else None)
+            p.form = OrderPositionChangeForm(
+                prefix='op-{}'.format(p.pk),
+                instance=p,
+                items=self.items,
+                initial={'seat': p.seat.seat_guid if p.seat else None},
+                data=self.request.POST if self.request.method == "POST" else None
+            )
         return positions
 
     def get_context_data(self, **kwargs):