Fix API bug: Do not delete SubEventItems on PATCH request

src/pretix/api/serializers/event.py

@@ -459,12 +459,13 @@ class SubEventSerializer(I18nAwareModelSerializer):
 
     @transaction.atomic
     def update(self, instance, validated_data):
-        item_price_overrides_data = validated_data.pop('subeventitem_set') if 'subeventitem_set' in validated_data else {}
+        item_price_overrides_data = validated_data.pop('subeventitem_set')
-        variation_price_overrides_data = validated_data.pop('subeventitemvariation_set') if 'subeventitemvariation_set' in validated_data else {}
+        variation_price_overrides_data = validated_data.pop('subeventitemvariation_set')
         meta_data = validated_data.pop('meta_data', None)
         seat_category_mapping = validated_data.pop('seat_category_mapping', None)
         subevent = super().update(instance, validated_data)
 
+        if item_price_overrides_data is not None:
             existing_item_overrides = {item.item: item.id for item in SubEventItem.objects.filter(subevent=subevent)}
 
             for item_price_override_data in item_price_overrides_data:
@@ -473,6 +474,7 @@ class SubEventSerializer(I18nAwareModelSerializer):
 
             SubEventItem.objects.filter(id__in=existing_item_overrides.values()).delete()
 
+        if variation_price_overrides_data is not None:
             existing_variation_overrides = {item.variation: item.id for item in SubEventItemVariation.objects.filter(subevent=subevent)}
 
             for variation_price_override_data in variation_price_overrides_data:

src/tests/api/test_subevents.py

@@ -562,6 +562,37 @@ def test_subevent_update(token_client, organizer, event, subevent, item, item2,
     assert resp.content.decode() == '{"non_field_errors":["One or more variations do not belong to this event."]}'
 
 
+@pytest.mark.django_db
+def test_subevent_update_keep_subeventitems(token_client, organizer, event, subevent, item, item2):
+    resp = token_client.patch(
+        '/api/v1/organizers/{}/events/{}/subevents/{}/'.format(organizer.slug, event.slug, subevent.pk),
+        {
+            "item_price_overrides": [
+                {
+                    "item": item.pk,
+                    "price": "88.88",
+                    "disabled": True
+                }
+            ],
+        },
+        format='json'
+    )
+    assert resp.status_code == 200
+    with scopes_disabled():
+        assert event.subevents.get(id=subevent.id).item_overrides[item.pk].disabled
+
+    resp = token_client.patch(
+        '/api/v1/organizers/{}/events/{}/subevents/{}/'.format(organizer.slug, event.slug, subevent.pk),
+        {
+            "date_from": "2017-12-27T10:00:00Z",
+        },
+        format='json'
+    )
+    assert resp.status_code == 200
+    with scopes_disabled():
+        assert event.subevents.get(id=subevent.id).item_overrides[item.pk].disabled
+
+
 @pytest.mark.django_db
 def test_subevent_detail(token_client, organizer, event, subevent):
     res = dict(TEST_SUBEVENT_RES)