Add Order.cancellation_date (#1629)

* Add Order.cancellation_date

* Add tests

src/pretix/base/migrations/0149_order_cancellation_date.py

@@ -0,0 +1,46 @@
+# Generated by Django 3.0.4 on 2020-03-25 14:40
+
+from django.db import migrations, models
+from django.db.models import OuterRef, Count, Subquery, Q
+from django.utils.timezone import now
+
+
+def fill_cancellation_date(apps, schema_editor):
+    Order = apps.get_model('pretixbase', 'Order')
+    LogEntry = apps.get_model('pretixbase', 'LogEntry')
+    OrderPosition = apps.get_model('pretixbase', 'OrderPosition')
+
+    s = OrderPosition.all.filter(
+        order=OuterRef('pk'),
+        canceled=False,
+    ).order_by().values('order').annotate(k=Count('id')).values('k')
+    for o in Order.objects.annotate(
+            pcnt=Subquery(s)
+    ).filter(
+        Q(pcnt=0) | Q(pcnt__isnull=True) | Q(status="c")
+    ).values('id').iterator():
+        le = LogEntry.objects.filter(
+            content_type__model="order",
+            object_id=o['id'],
+            action_type='pretix.event.order.canceled'
+        ).order_by('-datetime').only('datetime').first()
+        if le:
+            Order.objects.filter(pk=o['id']).update(
+                cancellation_date=le.datetime,
+                last_modified=now()
+            )
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('pretixbase', '0148_cancellationrequest'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='order',
+            name='cancellation_date',
+            field=models.DateTimeField(blank=True, null=True),
+        ),
+        migrations.RunPython(fill_cancellation_date, migrations.RunPython.noop)
+    ]