Fix #1080 -- Deal with gaps in the invoice database (#1086)

src/pretix/base/models/invoices.py

@@ -2,6 +2,8 @@ import string
 from decimal import Decimal
 
 from django.db import DatabaseError, models, transaction
+from django.db.models import Max
+from django.db.models.functions import Cast
 from django.utils import timezone
 from django.utils.crypto import get_random_string
 from django.utils.functional import cached_property
@@ -124,8 +126,12 @@ class Invoice(models.Model):
         numeric_invoices = Invoice.objects.filter(
             event__organizer=self.event.organizer,
             prefix=self.prefix,
-        ).exclude(invoice_no__contains='-')
+        ).exclude(invoice_no__contains='-').annotate(
-        return self._to_numeric_invoice_number(numeric_invoices.count() + 1)
+            numeric_number=Cast('invoice_no', models.IntegerField())
+        ).aggregate(
+            max=Max('numeric_number')
+        )['max'] or 0
+        return self._to_numeric_invoice_number(numeric_invoices + 1)
 
     def _get_invoice_number_from_order(self):
         return '{order}-{count}'.format(
@@ -183,7 +189,7 @@ class Invoice(models.Model):
 
     class Meta:
         unique_together = ('organizer', 'prefix', 'invoice_no')
-        ordering = ('invoice_no',)
+        ordering = ('date', 'invoice_no',)
 
 
 class InvoiceLine(models.Model):

src/tests/base/test_invoices.py

@@ -294,20 +294,27 @@ def test_invoice_numbers(env):
 
     event.settings.set('invoice_numbers_consecutive', True)
     inv5 = generate_invoice(order)
+    inv6 = generate_invoice(order)
+    inv7 = generate_invoice(order)
+    Invoice.objects.filter(pk=inv6.pk).delete()  # This should never ever happen, but what if it happens anyway?
+    inv8 = generate_invoice(order)
     inv23 = generate_invoice(order2)
 
-    # expected behaviour for switching between numbering formats
+    # expected behaviour for switching between numbering formats or dealing with gaps
     assert inv1.invoice_no == '00001'
     assert inv2.invoice_no == '00002'
     assert inv3.invoice_no == '{}-3'.format(order.code)
     assert inv4.invoice_no == '{}-4'.format(order.code)
     assert inv5.invoice_no == '00003'
+    assert inv6.invoice_no == '00004'
+    assert inv7.invoice_no == '00005'
+    assert inv8.invoice_no == '00006'
 
     # test that separate orders are counted separately in this mode
     assert inv21.invoice_no == '{}-1'.format(order2.code)
     assert inv22.invoice_no == '{}-2'.format(order2.code)
     # but consecutively in this mode
-    assert inv23.invoice_no == '00004'
+    assert inv23.invoice_no == '00007'
 
     # test Invoice.number, too
     assert inv1.number == '{}-00001'.format(event.slug.upper())