Avoid conflict with self on validation

Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/billing/models.py

@@ -170,7 +170,15 @@ class Invoice(models.Model):
         overlapping = Invoice.objects.filter(
             (Q(start__lte=self.end) & Q(end__gte=self.end)) |
             (Q(start__lte=self.start) & Q(end__gte=self.start))
-        ).filter(billing=self.billing)
+        ).filter(
+            billing=self.billing
+        )
+
+        if self.pk:
+            overlapping = overlapping.exclude(
+                pk=self.pk
+            )
+
         if overlapping.exists():
             raise ValidationError(
                 'Overlapping invoices exist: {0}'.format(overlapping)

weblate/billing/tests.py

@@ -122,3 +122,6 @@ class BillingTest(TestCase):
         # Valid before existing
         invoice.end = self.invoice.start - timedelta(days=1)
         invoice.clean()
+
+        # Validation of existing
+        self.invoice.clean()