Dashboard: optimize projects queryset (#12383)

This used to be 35 queries. I found another optimization to make it to
constant time, but will open another PR as it may produce a slow query..

Author: stsewd

readthedocs/projects/querysets.py

@@ -144,13 +144,11 @@ def prefetch_latest_build(self):
 
         # Prefetch the latest build for each project.
         subquery = Subquery(
-            Build.internal.filter(project=OuterRef("project_id"))
-            .order_by("-date")
-            .values_list("id", flat=True)[:1]
+            Build.internal.filter(project=OuterRef("project_id")).order_by("-date").values("pk")[:1]
         )
         latest_build = Prefetch(
             "builds",
-            Build.internal.filter(pk__in=subquery),
+            Build.objects.filter(pk__in=subquery).select_related("version"),
             to_attr=self.model.LATEST_BUILD_CACHE,
         )
         return self.prefetch_related(latest_build)

readthedocs/rtd_tests/tests/test_project_views.py

@@ -1,8 +1,6 @@
 from unittest import mock
 
-import pytest
 from allauth.socialaccount.models import SocialAccount
-from django.conf import settings
 from django.contrib.auth.models import User
 from django.http.response import HttpResponseRedirect
 from django.test import TestCase, override_settings
@@ -380,6 +378,30 @@ def setUp(self):
         self.client.login(username="eric", password="test")
         self.project = get(Project, slug="pip", users=[self.user])
 
+    def test_dashboard_number_of_queries(self):
+        for i in range(10):
+            project = get(
+                Project,
+                slug=f"project-{i}",
+                users=[self.user],
+            )
+            version = project.versions.first()
+            version.active = True
+            version.built = True
+            version.save()
+            for _ in range(3):
+                get(
+                    Build,
+                    project=project,
+                    version=version,
+                    success=True,
+                    state=BUILD_STATE_FINISHED,
+                )
+
+        with self.assertNumQueries(24):
+            r = self.client.get(reverse(("projects_dashboard")))
+        assert r.status_code == 200
+
     def test_versions_page(self):
         self.project.versions.create(verbose_name="1.0")