fix(admin): handle unsaved performance seats display

backend/shows/admin.py

@@ -41,4 +41,11 @@ class PerformanceAdmin(admin.ModelAdmin):
 
     @admin.display(description="Available seats")
     def available_seats_display(self, obj):
+        if (
+            not getattr(obj, "pk", None)
+            or obj.room_capacity is None
+            or obj.additional_seats is None
+            or obj.manually_occupied_seats is None
+        ):
+            return "-"
         return obj.available_seats()

backend/shows/test_admin.py

@@ -1,5 +1,7 @@
 from django.contrib import admin
-from django.test import SimpleTestCase
+from django.contrib.auth import get_user_model
+from django.test import SimpleTestCase, TestCase
+from django.urls import reverse
 
 from bookings.models import Reservation, ReservationToken
 from checkins.models import CheckIn
@@ -11,3 +13,20 @@ class AdminRegistrationTests(SimpleTestCase):
         for model in (Show, Venue, Performance, Reservation, ReservationToken, CheckIn):
             with self.subTest(model=model.__name__):
                 self.assertTrue(admin.site.is_registered(model))
+
+
+class PerformanceAdminTests(TestCase):
+    def setUp(self):
+        user_model = get_user_model()
+        self.admin_user = user_model.objects.create_superuser(
+            username="admin",
+            email="admin@example.com",
+            password="password123",
+        )
+        self.client.force_login(self.admin_user)
+
+    def test_performance_add_page_renders_for_unsaved_object(self):
+        response = self.client.get(reverse("admin:shows_performance_add"))
+
+        self.assertEqual(response.status_code, 200)
+        self.assertContains(response, "Available seats")