Fix #634 -- Do not allow deleting the last date of an event series (#675)

* Checking for the last date in the event series before deleting a date. Last date in a event series should never be delted.

* Adding check to ensure that last date in a event series is not deleted. Editing unit test around deleting subevent to assert on alert-danger

* Increasing the scope of test_delete. We are now creating 2 subevents and testing deleting one and ensuring that the last one is not deleted

* Fixing alert text. Removing a redundant if condition for checking subevent count

* Adding assert for second event to ensure its not deleted

* Minor fixes and rebase

src/pretix/base/models/event.py

@@ -639,6 +639,9 @@ class SubEvent(EventMixin, LoggedModel):
         data.update({v.property.name: v.value for v in self.meta_values.select_related('property').all()})
         return data
 
+    def allow_delete(self):
+        return self.event.subevents.count() > 1
+
     def delete(self, *args, **kwargs):
         super().delete(*args, **kwargs)
         if self.event:

src/pretix/control/views/subevents.py

@@ -101,10 +101,13 @@ class SubEventDelete(EventPermissionRequiredMixin, DeleteView):
         self.object = self.get_object()
         success_url = self.get_success_url()
 
-        if self.get_object().orderposition_set.count() > 0:
+        if self.object.orderposition_set.count() > 0:
             messages.error(request, pgettext_lazy('subevent', 'A date can not be deleted if orders already have been '
                                                   'placed.'))
             return HttpResponseRedirect(self.get_success_url())
+        elif not self.object.allow_delete():  # checking if this is the last date in the event series
+            messages.error(request, pgettext_lazy('subevent', 'The last date of an event series can not be deleted.'))
+            return HttpResponseRedirect(self.get_success_url())
         else:
             self.object.log_action('pretix.subevent.deleted', user=self.request.user)
             self.object.delete()

src/tests/control/test_events.py

@@ -8,7 +8,7 @@ from pytz import timezone
 from tests.base import SoupTest, extract_form_fields
 
 from pretix.base.models import (
-    Event, Order, OrderPosition, Organizer, Team, User,
+    Event, Order, OrderPosition, Organizer, SubEvent, Team, User,
 )
 from pretix.base.models.items import SubEventItem
 from pretix.testutils.mock import mocker_context
@@ -594,6 +594,7 @@ class SubEventsTest(SoupTest):
         self.client.login(email='dummy@dummy.dummy', password='dummy')
 
         self.subevent1 = self.event1.subevents.create(name='SE1', date_from=now())
+        self.subevent2 = self.event1.subevents.create(name='SE2', date_from=now())
 
     def test_list(self):
         doc = self.get_doc('/control/event/ccc/30c3/subevents/')
@@ -699,7 +700,11 @@ class SubEventsTest(SoupTest):
         assert doc.select("button")
         doc = self.post_doc('/control/event/ccc/30c3/subevents/%d/delete' % self.subevent1.pk, {})
         assert doc.select(".alert-success")
-        assert not SubEventItem.objects.filter(pk=self.subevent1.pk).exists()
+        # deleting the second event
+        doc = self.post_doc('/control/event/ccc/30c3/subevents/%d/delete' % self.subevent2.pk, {})
+        assert doc.select(".alert-danger")
+        assert SubEvent.objects.filter(pk=self.subevent2.pk).exists()
+        assert not SubEvent.objects.filter(pk=self.subevent1.pk).exists()
 
     def test_delete_with_orders(self):
         o = Order.objects.create(