Allow customers to change add-ons on existing orders (#2283)

src/pretix/base/migrations/0203_orderposition_is_bundled.py

@@ -0,0 +1,17 @@
+# Generated by Django 3.2.2 on 2021-11-08 07:51
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('pretixbase', '0202_user_needs_password_change'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='orderposition',
+            name='is_bundled',
+            field=models.BooleanField(default=False),
+        ),
+    ]

src/pretix/base/migrations/0204_orderposition_backfill_is_bundled.py

@@ -0,0 +1,46 @@
+# Generated by Django 3.2.2 on 2021-11-08 07:51
+
+from django.db import migrations, models
+from django.db.models import Count, OuterRef, Subquery
+from django.db.models.functions import Coalesce
+
+
+def fill_is_bundled(apps, schema_editor):
+    # We cannot really know if a position was bundled or an add-on, but we can at least guess
+    ItemBundle = apps.get_model("pretixbase", "ItemBundle")
+    OrderPosition = apps.get_model("pretixbase", "OrderPosition")
+
+    for ib in ItemBundle.objects.iterator():
+        OrderPosition.all.alias(
+            pos_earlier=Coalesce(Subquery(
+                OrderPosition.all.filter(
+                    canceled=False,
+                    addon_to=OuterRef('addon_to'),
+                    item=ib.bundled_item,
+                    variation=ib.bundled_variation,
+                    positionid__lt=OuterRef('positionid'),
+                ).values('addon_to').order_by().annotate(c=Count('*')).values('c'),
+                output_field=models.IntegerField()
+            ), 0)
+        ).filter(
+            canceled=False,
+            addon_to__item=ib.base_item,
+            item=ib.bundled_item,
+            variation=ib.bundled_variation,
+            pos_earlier__lt=ib.count,
+        ).update(
+            is_bundled=True
+        )
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('pretixbase', '0203_orderposition_is_bundled'),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            fill_is_bundled,
+            migrations.RunPython.noop,
+        ),
+    ]