Team Number : Team 146

Linked Issue

Closes #34

Problem

getDashboard and getTodayStatus in dashboard.controller.js suffered from a severe N+1 query problem. For every active membership, two separate database queries were fired inside a Promise.all(memberships.map(...)) loop:

• prisma.dailyResult.findUnique — one per membership for today's result
• evaluationService.getMemberDailyResults → another prisma.dailyResult.findMany per membership for recent results

For a user in 15 challenges, this produced 30+ database queries per single API request, rapidly exhausting the PostgreSQL connection pool under load.

Solution

Refactored both endpoints to query the database in bulk and group data in memory, following standard ORM performance best practices.

Changes — src/services/evaluation.service.js

• Added normaliseToMidnight() — a shared pure helper that normalises any Date to local midnight, eliminating repeated inline date mutation across the codebase.
• Added getBulkTodayResults(memberIds) — fetches today's dailyResult rows for all members in one query, returns a { [memberId]: result } map for O(1) lookup.
• Added getBulkMemberDailyResults(memberIds, daysBack) — fetches the last N calendar days of results for all members in one query, returns a { [memberId]: result[] } map. Uses a calendar window (not take: N) so missed days are correctly represented as absent entries in the activity strip.

Changes — src/controllers/dashboard.controller.js

• getDashboard: removed all inline Prisma queries, date normalization, and reduce grouping. Delegates entirely to getBulkTodayResults + getBulkMemberDailyResults via Promise.all. Response mapping is now purely in-memory.
• getTodayStatus: same treatment — removed the per-membership findUnique loop, replaced with a single call to getBulkTodayResults.
• Added early-return guard for empty memberships in both endpoints.

Query Reduction

Testing

── normaliseToMidnight ──────────────────────────────────────
✅ hours set to 0
✅ minutes set to 0
✅ seconds set to 0
✅ ms set to 0
✅ returns a new Date (no mutation)
✅ original date is not mutated

── getBulkTodayResults — in-memory grouping ─────────────────
✅ member A has a today result
✅ member A completed today
✅ member B has a today result
✅ member B did not complete today
✅ member C has no today result (absent = missed day)
✅ empty rows → empty map

── getBulkMemberDailyResults — in-memory grouping ───────────
✅ member A has 3 recent entries
✅ member B has 1 recent entry
✅ member C absent (no results in window)
✅ member A results are ordered newest-first
✅ empty rows → empty map

── getBulkAllMemberResults — leaderboard aggregation ────────
✅ member A totalDays = 3
✅ member A completedDays = 2
✅ member B totalDays = 3
✅ member B completedDays = 1
✅ member C absent → no entry
✅ member A completionRate = 66.67 (expected 66.67)
✅ empty rows → empty map

── getDashboard — controller response shaping ───────────────
✅ returns entry for every membership
✅ member A todayStatus populated
✅ member A todayStatus.completed correct
✅ member A has 3 recent results
✅ member B todayStatus populated
✅ member B todayStatus.completed correct
✅ member C todayStatus is null (no result)
✅ member C recentResults is empty array

── getTodayStatus — controller response shaping ─────────────
✅ returns entry for every membership
✅ member A challengeId correct
✅ member A status populated
✅ member A status.completed correct
✅ member A submissionsCount correct
✅ member B status.completed correct
✅ member C status is null

── getChallengeLeaderboard — response shaping ───────────────
✅ returns entry for every member
✅ alice is first (order preserved from DB sort)
✅ alice totalDays = 3
✅ alice completedDays = 2
✅ alice completionRate = 66.67
✅ bob completedDays = 1
✅ bob completionRate = 33.33
✅ carol totalDays = 0 (no results)
✅ carol completionRate = 0.00
✅ carol leetcodeUsername null handled safely

── Edge cases ───────────────────────────────────────────────
✅ empty memberships array triggers early return
✅ groupTodayResults([]) → {}
✅ groupRecentResults([]) → {}
✅ aggregateAllResults([]) → {}
✅ last row wins for duplicate memberId (consistent with DB unique constraint)

────────────────────────────────────────────────────────────
Results: 54 passed, 0 failed
ALL TESTS PASSED ✅