fix: eliminate N+1 queries in ContestList scoreboard visibility check

judge/models/contest.py

@@ -267,7 +267,11 @@ def can_see_full_scoreboard(self, user):
             return True
         if user.profile.id in self.editor_ids:
             return True
-        if self.view_contest_scoreboard.filter(id=user.profile.id).exists():
+        cache = getattr(self, '_prefetched_objects_cache', {})
+        if 'view_contest_scoreboard' in cache:
+            if any(p.id == user.profile.id for p in self.view_contest_scoreboard.all()):
+                return True
+        elif self.view_contest_scoreboard.filter(id=user.profile.id).exists():
             return True
         if self.scoreboard_visibility == self.SCOREBOARD_AFTER_PARTICIPATION and self.has_completed_contest(user):
             return True
@@ -377,12 +381,27 @@ def ended(self):
 
     @cached_property
     def author_ids(self):
+        cache = getattr(self, '_prefetched_objects_cache', {})
+        if 'authors' in cache:
+            return frozenset(p.id for p in self.authors.all())
         return Contest.authors.through.objects.filter(contest=self).values_list('profile_id', flat=True)
 
     @cached_property
     def editor_ids(self):
-        return self.author_ids.union(
-            Contest.curators.through.objects.filter(contest=self).values_list('profile_id', flat=True))
+        cache = getattr(self, '_prefetched_objects_cache', {})
+        if 'authors' in cache:
+            author_ids = frozenset(p.id for p in self.authors.all())
+        else:
+            author_ids = frozenset(
+                Contest.authors.through.objects.filter(contest=self).values_list('profile_id', flat=True),
+            )
+
+        if 'curators' in cache:
+            return author_ids | frozenset(p.id for p in self.curators.all())
+
+        return author_ids | frozenset(
+            Contest.curators.through.objects.filter(contest=self).values_list('profile_id', flat=True),
+        )
 
     @cached_property
     def tester_ids(self):

judge/tests/test_contest_list_perf.py

@@ -0,0 +1,51 @@
+from datetime import timedelta
+
+from django.contrib.auth import get_user_model
+from django.db import connection, reset_queries
+from django.test import Client, TestCase, override_settings
+from django.utils import timezone
+
+from judge.models import Contest, Profile
+
+User = get_user_model()
+
+NUM_CONTESTS = 10
+QUERY_BUDGET = 35
+
+
+class ContestListPerfTest(TestCase):
+    """Ensure ContestList does not generate N+1 queries for hidden-scoreboard contests."""
+
+    @classmethod
+    def setUpTestData(cls):
+        cls.user = User.objects.create_user(username='perf_user', password='pass')
+        Profile.objects.create(user=cls.user)
+
+        now = timezone.now()
+        for i in range(NUM_CONTESTS):
+            Contest.objects.create(
+                key=f'hidden_contest_{i}',
+                name=f'Hidden Contest {i}',
+                start_time=now - timedelta(hours=2),
+                end_time=now - timedelta(hours=1),
+                is_visible=True,
+                scoreboard_visibility=Contest.SCOREBOARD_HIDDEN,
+            )
+
+    def setUp(self):
+        self.client = Client()
+        self.client.login(username='perf_user', password='pass')
+
+    def test_query_count_bounded(self):
+        """Query count must stay within budget regardless of number of hidden-scoreboard contests."""
+        with override_settings(DEBUG=True):
+            reset_queries()
+            response = self.client.get('/contests/')
+            query_count = len(connection.queries)
+
+        self.assertEqual(response.status_code, 200)
+        print(f'\n  [{NUM_CONTESTS} hidden-scoreboard contests] query count: {query_count} (budget: {QUERY_BUDGET})')
+        self.assertLessEqual(
+            query_count, QUERY_BUDGET,
+            msg=f'Expected <= {QUERY_BUDGET} queries for {NUM_CONTESTS} contests, got {query_count}.',
+        )

judge/views/contests.py

@@ -104,7 +104,9 @@ def _now(self):
         return timezone.now()
 
     def _get_queryset(self):
-        return super().get_queryset().prefetch_related('tags', 'organization', 'authors', 'curators', 'testers')
+        return super().get_queryset().prefetch_related(
+            'tags', 'organization', 'authors', 'curators', 'testers', 'view_contest_scoreboard',
+        )
 
     def get_queryset(self):
         self.search_query = None
@@ -133,7 +135,8 @@ def get_context_data(self, **kwargs):
             for participation in ContestParticipation.objects.filter(virtual=0, user=self.request.profile,
                                                                      contest_id__in=present) \
                     .select_related('contest') \
-                    .prefetch_related('contest__authors', 'contest__curators', 'contest__testers') \
+                    .prefetch_related('contest__authors', 'contest__curators', 'contest__testers',
+                                      'contest__view_contest_scoreboard') \
                     .annotate(key=F('contest__key')):
                 if participation.ended:
                     finished.add(participation.contest.key)