Add test for cache invalidation and fix test cache clearing

Co-authored-by: alexeygrigorev <875246+alexeygrigorev@users.noreply.github.com>

Author: Copilot

courses/tests/test_course.py

@@ -1,6 +1,7 @@
 from django.test import TestCase, Client
 from django.urls import reverse
 from django.utils import timezone
+from django.core.cache import cache
 
 from courses.models import (
     User,
@@ -27,6 +28,9 @@
 
 class CourseDetailViewTests(TestCase):
     def setUp(self):
+        # Clear cache before each test to ensure fresh state
+        cache.clear()
+        
         self.client = Client()
 
         self.user = User.objects.create_user(**credentials)

courses/tests/test_leaderboard.py

@@ -3,6 +3,7 @@
 from django.test import TestCase
 from django.utils import timezone
 from datetime import timedelta
+from django.core.cache import cache
 
 from courses.models import (
     Course,
@@ -125,3 +126,29 @@ def test_leaderboard(self):
             self.assertEqual(enrollment.position_on_leaderboard, rank)
 
             self.assertEqual(enrollment.total_score, score)
+
+    def test_leaderboard_cache_invalidation(self):
+        """Test that leaderboard cache is invalidated when update_leaderboard is called"""
+        # Create some test data
+        enrollment1 = self.create_student("student1")
+        enrollment2 = self.create_student("student2")
+
+        homework = self.create_homework(1)
+        self.submit_homework(homework, enrollment1, score=100)
+        self.submit_homework(homework, enrollment2, score=50)
+
+        # Update leaderboard (this should populate the cache)
+        update_leaderboard(self.course)
+
+        # Check the cache key
+        cache_key = f"leaderboard:{self.course.id}"
+
+        # Manually set a value in cache to verify it gets invalidated
+        cache.set(cache_key, "test_value", 3600)
+        self.assertEqual(cache.get(cache_key), "test_value")
+
+        # Update leaderboard again (this should invalidate the cache)
+        update_leaderboard(self.course)
+
+        # Cache should be invalidated (None)
+        self.assertIsNone(cache.get(cache_key))