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Optimize N+1 Queries Across API Endpoints #3333

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Optimize N+1 Queries Across API Endpoints #3333

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1065bdd
fix(perf): Optimize N+1 queries across API endpoints
blarghmatey Feb 24, 2026
8bb75b8
fix: Remove invalid prefetch_related calls that don't resolve to actu…
blarghmatey Feb 25, 2026
0aa5955
fix: Restore custom prefetch for enrollment certificate and grades
blarghmatey Feb 25, 2026
944419d
refactor: Revert program enrollments N+1 optimization in favor of sim…
blarghmatey Feb 25, 2026
803a068
Fix prefetch_related in ContractPageViewSet to use correct relationship
blarghmatey Feb 25, 2026
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4 changes: 3 additions & 1 deletion b2b/views/v0/__init__.py
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Original file line number Diff line number Diff line change
Expand Up @@ -54,7 +54,9 @@ class ContractPageViewSet(viewsets.ReadOnlyModelViewSet):

def get_queryset(self):
"""Filter to only return active contracts by default."""
return ContractPage.objects.filter(active=True)
return ContractPage.objects.filter(active=True).prefetch_related(
"contract_programs"
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Copilot AI Feb 25, 2026

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Prefetching contract_programs here won’t eliminate the N+1 in ContractPageSerializer.get_programs, because that method calls instance.programs (a @property that runs a fresh Program.objects.filter(...) query per contract). To actually reduce queries, prefetch the relationship that the property uses (e.g., contract_programs__program) and/or change the serializer to read program IDs from the prefetched contract_programs items instead of calling instance.programs.

Suggested change
"contract_programs"
"contract_programs",
"contract_programs__program",

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)


class Enroll(APIView):
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14 changes: 8 additions & 6 deletions courses/views/v1/__init__.py
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Original file line number Diff line number Diff line change
Expand Up @@ -244,21 +244,23 @@ def get_queryset(self):
if relevant_to:
course = Course.objects.filter(readable_id=relevant_to).first()
if course:
return get_relevant_course_run_qset(course)
return get_relevant_course_run_qset(course).prefetch_related("products")
else:
program = Program.objects.filter(readable_id=relevant_to).first()
return (
get_user_relevant_program_course_run_qset(program)
if program
else Program.objects.none()
)
if program:
return get_user_relevant_program_course_run_qset(
program
).prefetch_related("products")
else:
return CourseRun.objects.none()
else:
return (
CourseRun.objects.select_related("course")
.prefetch_related(
"course__departments",
"course__page",
"enrollment_modes",
"products",
)
.filter(live=True)
)
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7 changes: 6 additions & 1 deletion courses/views/v2/__init__.py
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Original file line number Diff line number Diff line change
Expand Up @@ -337,7 +337,12 @@ def filter_queryset(self, queryset):

if "courserun_is_enrollable" not in filter_keys:
queryset = queryset.prefetch_related(
Prefetch("courseruns", queryset=CourseRun.objects.order_by("id")),
Prefetch(
"courseruns",
queryset=CourseRun.objects.order_by("id").prefetch_related(
"products"
),
),
)
Comment on lines 339 to 346
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Copilot AI Feb 25, 2026

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This Prefetch("courseruns", queryset=...prefetch_related("products")) is unlikely to help if downstream serialization re-queries instance.courseruns with .order_by(...) / .filter(...) (which bypasses the prefetched cache). In CourseWithCourseRunsSerializer.get_courseruns the related manager is queried again, so products may still be fetched N+1. Consider either (1) moving the products prefetch onto the queryset the serializer actually uses, or (2) using to_attr and having the serializer consume the prefetched list without re-querying.

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return queryset
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