perf: eliminate source health N plus one queries

apps/sources/services/health.py

@@ -4,6 +4,7 @@ from collections.abc import Iterable
 from dataclasses import dataclass
 from datetime import datetime, timedelta
 
+from django.db.models import Prefetch
 from django.utils import timezone
 
 from apps.sources.models import Source, SourcePolicyReview, SourceRun
@@ -214,12 +215,17 @@ def _determine_health_action(runs: list[SourceRun]) -> tuple[str | None, str | N
 def collect_source_health(
     *, runs_to_consider: int = SOURCE_HEALTH_RUN_WINDOW
 ) -> list[SourceHealth]:
-    sources = Source.objects.order_by("name").all()
+    sources = Source.objects.prefetch_related(
+        Prefetch(
+            "runs",
+            queryset=SourceRun.objects.order_by("-started_at")[:runs_to_consider],
+            to_attr="health_runs",
+        )
+    ).order_by("name")
     rows: list[SourceHealth] = []
 
     for source in sources:
-        run_queryset = SourceRun.objects.filter(source=source).order_by("-started_at")
+        runs = source.health_runs
-        runs = list(run_queryset[:runs_to_consider])
         monitored_runs = len(runs)
         success_runs = [run for run in runs if run.status == SourceRun.Status.SUCCESS]
 

tests/integration/test_source_health.py

@@ -111,6 +111,25 @@ def test_collect_source_health_exposes_parser_and_counts(source):
     assert row.updated_count == 1
 
 
+@pytest.mark.django_db
+def test_collect_source_health_prefetches_runs_for_all_sources(source, django_assert_num_queries):
+    second_source = Source.objects.create(
+        name="Second jobs",
+        source_type=Source.Type.EMPLOYER,
+        base_url="https://second.example.org/jobs/",
+        domain="second.example.org",
+        status=Source.Status.ACTIVE,
+        policy=Source.Policy.ALLOW,
+    )
+    _create_run(source, status=SourceRun.Status.SUCCESS)
+    _create_run(second_source, status=SourceRun.Status.SUCCESS)
+
+    with django_assert_num_queries(2):
+        rows = collect_source_health()
+
+    assert [row.source_name for row in rows] == ["Example jobs", "Second jobs"]
+
+
 @pytest.mark.django_db
 def test_recover_source_health_starts_canary_once(source, monkeypatch):
     source.status = Source.Status.QUARANTINED