Cache leaderboard data to improve page load performance (#148)

* Initial plan

* Implement leaderboard caching with cache invalidation

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

* Add test for cache invalidation and fix test cache clearing

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

* Address code review feedback: cache dictionaries instead of model instances

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

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: alexeygrigorev <875246+alexeygrigorev@users.noreply.github.com>

Author: Copilot

course_management/settings.py

@@ -186,6 +186,14 @@
 
 VERSION = os.getenv("VERSION", "N/A")
 
+# Cache configuration
+CACHES = {
+    "default": {
+        "BACKEND": "django.core.cache.backends.locmem.LocMemCache",
+        "LOCATION": "default-cache",
+    }
+}
+
 # authentication
 
 AUTHENTICATION_BACKENDS = (

courses/scoring.py

@@ -9,6 +9,7 @@
 from django.db.models import Sum, Count
 
 from django.db import transaction
+from django.core.cache import cache
 
 
 from .models import (
@@ -345,6 +346,11 @@ def update_leaderboard(course: Course):
         ["total_score", "position_on_leaderboard"],
     )
 
+    # Invalidate the leaderboard cache
+    cache_key = f"leaderboard:{course.id}"
+    cache.delete(cache_key)
+    logger.info(f"Invalidated cache for leaderboard of course {course.id}")
+
     t1 = time()
     logger.info(f"Updated leaderboard in {(t1 - t0):.2f} seconds")
 

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)
@@ -313,7 +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)
 
@@ -349,13 +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)
 
@@ -390,7 +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

@@ -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,
@@ -58,6 +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",
@@ -125,3 +129,29 @@ 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))

courses/views/course.py

@@ -14,6 +14,7 @@
 from django.db.models.functions import Coalesce
 
 from django.contrib.auth.decorators import login_required
+from django.core.cache import cache
 
 from courses.models import (
     Course,
@@ -258,13 +259,39 @@ def leaderboard_view(request, course_slug: str):
         except Enrollment.DoesNotExist:
             pass
 
-    enrollments = Enrollment.objects.filter(course=course).order_by(
-        Coalesce("position_on_leaderboard", Value(999999)),
-        "id",
-    )
+    # Try to get enrollments from cache
+    cache_key = f"leaderboard:{course.id}"
+    enrollments_data = cache.get(cache_key)
+
+    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)
+            .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_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,