Address code review feedback: cache dictionaries instead of model instances

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

Author: Copilot

courses/tests/test_course.py

@@ -30,7 +30,7 @@ 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)
@@ -317,7 +317,7 @@ def test_leaderboard_order(self):
             self.enrollment.display_name,
         ]
 
-        actual_order = [e.display_name for e in enrollments]
+        actual_order = [e['display_name'] for e in enrollments]
 
         self.assertEqual(actual_order, expected_order)
 
@@ -353,13 +353,13 @@ def test_new_enrollment_at_the_end_of_leaderboard(self):
             e5.display_name,
         ]
 
-        actual_order = [e.display_name for e in enrollments]
+        actual_order = [e['display_name'] for e in enrollments]
 
         self.assertEqual(actual_order, expected_order)
 
         expected_positions = [1, 2, 3, 4, None, None]
         actual_positions = [
-            e.position_on_leaderboard for e in enrollments
+            e['position_on_leaderboard'] for e in enrollments
         ]
         self.assertEqual(actual_positions, expected_positions)
 
@@ -394,7 +394,7 @@ def test_not_enrolled_yet_but_leaderboard_displays(self):
 
         # Verify the order is correct
         expected_order = ["e1", "e2", "e3", "e4", "e5"]
-        actual_order = [e.display_name for e in enrollments]
+        actual_order = [e['display_name'] for e in enrollments]
         self.assertEqual(actual_order, expected_order)
 
     def test_not_enrolled_but_can_edit_details(self):

courses/tests/test_leaderboard.py

@@ -59,6 +59,9 @@ def submit_homework(self, homework, enrollment, score):
         )
 
     def setUp(self):
+        # Clear cache before each test to ensure fresh state
+        cache.clear()
+        
         self.course = Course.objects.create(
             slug="test-course",
             title="Test Course",

courses/views/course.py

@@ -261,25 +261,37 @@ def leaderboard_view(request, course_slug: str):
 
     # Try to get enrollments from cache
     cache_key = f"leaderboard:{course.id}"
-    enrollments = cache.get(cache_key)
+    enrollments_data = cache.get(cache_key)
 
-    if enrollments is None:
+    if enrollments_data is None:
         logger.info(f"Cache miss for leaderboard of course {course.slug}")
         # Cache miss, fetch from database
         enrollments = list(
-            Enrollment.objects.filter(course=course).order_by(
+            Enrollment.objects.filter(course=course)
+            .select_related('student')
+            .order_by(
                 Coalesce("position_on_leaderboard", Value(999999)),
                 "id",
             )
         )
+        # Store as list of dictionaries to avoid stale model instances
+        enrollments_data = [
+            {
+                'id': e.id,
+                'display_name': e.display_name,
+                'total_score': e.total_score,
+                'position_on_leaderboard': e.position_on_leaderboard,
+            }
+            for e in enrollments
+        ]
         # Cache for 1 hour (3600 seconds)
         # The cache will be invalidated when leaderboard is recalculated
-        cache.set(cache_key, enrollments, 3600)
+        cache.set(cache_key, enrollments_data, 3600)
     else:
         logger.info(f"Cache hit for leaderboard of course {course.slug}")
 
     context = {
-        "enrollments": enrollments,
+        "enrollments": enrollments_data,
         "course": course,
         "current_student_enrollment": current_student_enrollment,
         "current_student_enrollment_id": current_student_enrollment_id,