return ids of occupying objects instead of boolean flags

src/pretix/api/serializers/event.py

@@ -972,19 +972,19 @@ class ItemMetaPropertiesSerializer(I18nAwareModelSerializer):
 
 
 class SeatSerializer(I18nAwareModelSerializer):
-    has_order = serializers.BooleanField()
-    has_cart = serializers.BooleanField()
-    has_voucher = serializers.BooleanField()
+    order = serializers.CharField()
+    cart = serializers.IntegerField()
+    voucher = serializers.IntegerField()
 
     class Meta:
         model = Seat
         read_only_fields = (
             'id', 'subevent', 'zone_name', 'row_name', 'row_label',
             'seat_number', 'seat_label', 'seat_guid', 'product', 'sorting_rank', 'x', 'y',
-            'has_order', 'has_cart', 'has_voucher',
+            'order', 'cart', 'voucher',
         )
         fields = (
             'id', 'subevent', 'zone_name', 'row_name', 'row_label',
             'seat_number', 'seat_label', 'seat_guid', 'product', 'blocked', 'sorting_rank', 'x', 'y',
-            'has_order', 'has_cart', 'has_voucher',
+            'order', 'cart', 'voucher',
         )

src/pretix/api/views/event.py

@@ -683,9 +683,9 @@ class SeatViewSet(ConditionalListView, viewsets.ModelViewSet):
                 subevent = self.request.event.subevents.get(pk=self.request.resolver_match.kwargs['subevent'])
             except SubEvent.DoesNotExist:
                 raise NotFound('Subevent not found')
-            return Seat.annotated(event_id=self.request.event.id, subevent=subevent, qs=subevent.seats.all())
+            return Seat.annotated(event_id=self.request.event.id, subevent=subevent, qs=subevent.seats.all(), annotate_ids=True)
         elif not self.request.event.has_subevents and 'subevent' not in self.request.resolver_match.kwargs:
-            return Seat.annotated(event_id=self.request.event.id, subevent=None, qs=self.request.event.seats.all())
+            return Seat.annotated(event_id=self.request.event.id, subevent=None, qs=self.request.event.seats.all(), annotate_ids=True)
         else:
             raise NotFound
 

src/pretix/base/models/seating.py

@@ -185,7 +185,7 @@ class Seat(models.Model):
 
     @classmethod
     def annotated(cls, qs, event_id, subevent, ignore_voucher_id=None, minimal_distance=0,
-                  ignore_order_id=None, ignore_cart_id=None, distance_only_within_row=False):
+                  ignore_order_id=None, ignore_cart_id=None, distance_only_within_row=False, annotate_ids=False):
         from . import CartPosition, Order, OrderPosition, Voucher
 
         vqs = Voucher.objects.filter(
@@ -214,17 +214,24 @@ class Seat(models.Model):
         )
         if ignore_cart_id:
             cqs = cqs.exclude(cart_id=ignore_cart_id)
-        qs_annotated = qs.annotate(
-            has_order=Exists(
-                opqs
-            ),
-            has_cart=Exists(
-                cqs
-            ),
-            has_voucher=Exists(
-                vqs
+        if annotate_ids:
+            qs_annotated = qs.annotate(
+                order=Subquery(opqs.values('order__code')),
+                cart=Subquery(cqs.values('id')),
+                voucher=Subquery(vqs.values('id')),
+            )
+        else:
+            qs_annotated = qs.annotate(
+                has_order=Exists(
+                    opqs
+                ),
+                has_cart=Exists(
+                    cqs
+                ),
+                has_voucher=Exists(
+                    vqs
+                )
             )
-        )
 
         if minimal_distance > 0:
             # TODO: Is there a more performant implementation on PostgreSQL using