SubEvent: Automatically bump all orders when date is changed

src/pretix/base/models/event.py

@@ -1113,12 +1113,28 @@ class SubEvent(EventMixin, LoggedModel):
         if self.event and clear_cache:
             self.event.cache.clear()
 
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.__original_dates = (self.date_from, self.date_to)
+
     def save(self, *args, **kwargs):
+        from .orders import Order
+
         clear_cache = kwargs.pop('clear_cache', False)
         super().save(*args, **kwargs)
         if self.event and clear_cache:
             self.event.cache.clear()
 
+        if (self.date_from, self.date_to) != self.__original_dates:
+            """
+            This is required to guarantee a synchronization invariant of our scanning apps.
+            Our syncing apps throw away order records of subevents more than X days ago, since
+            they are not interesting for ticket scanning and pose a performance hazard. However,
+            the app needs to know when a subevent is moved to a date in the future, since that
+            might require it to re-download and re-store the orders.
+            """
+            Order.objects.filter(all_positions__subevent=self).update(last_modified=now())
+
     @staticmethod
     def clean_items(event, items):
         for item in items:

src/tests/base/test_models.py

@@ -1,5 +1,6 @@
 import datetime
 import sys
+import time
 from datetime import date, timedelta
 from decimal import Decimal
 
@@ -2566,3 +2567,38 @@ def test_question_answer_validation_multiple_choice():
             assert q.clean_answer([o1.pk, o3.pk + 1000]) == [o1]
         with pytest.raises(ValidationError):
             assert q.clean_answer([o1.pk, 'FOO']) == [o1]
+
+
+@pytest.mark.django_db
+def test_subevent_date_updates_order_date():
+    # When the date of a subevent changes, all orders need to get a bumped modification date to hold
+    # a required invariant of the libpretixsync synchronization approach.
+    organizer = Organizer.objects.create(name='Dummy', slug='dummy')
+    with scope(organizer=organizer):
+        event = Event.objects.create(
+            organizer=organizer, name='Dummy', slug='dummy',
+            date_from=now(), date_to=now() - timedelta(hours=1), has_subevents=True
+        )
+        item1 = Item.objects.create(event=event, name="Ticket", default_price=23, admission=True)
+        se1 = event.subevents.create(date_from=now(), name="SE 1")
+        se2 = event.subevents.create(date_from=now(), name="SE 2")
+
+        order1 = Order.objects.create(event=event, status=Order.STATUS_PAID, expires=now() + timedelta(days=3), total=6)
+        OrderPosition.objects.create(order=order1, item=item1, subevent=se1, price=2)
+        order2 = Order.objects.create(event=event, status=Order.STATUS_PAID, expires=now() + timedelta(days=3), total=6)
+        OrderPosition.objects.create(order=order2, item=item1, subevent=se2, price=2)
+
+        o1lm = order1.last_modified
+        o2lm = order2.last_modified
+
+        time.sleep(1)
+        se1.date_from += timedelta(days=2)
+        se1.save()
+        se2.name = "foo"
+        se2.save()
+
+        order1.refresh_from_db()
+        order2.refresh_from_db()
+
+        assert order1.last_modified > o1lm
+        assert order2.last_modified == o2lm