Dashboard: optimize projects queryset (constant number of queries) (#12385)

This is on top of
#12383 as another PR
in case we see a regression in performance, it would be easier to just
revert this change.

Instead of pre-fetching, we annotate, so we can use exist instead of
fetching the query. This allows to just make one query. But this does
add some extra complexity to the query itself, which is less overhead
than doing a prefetch over the queryset itself, but the pagination
package we are using calls .count(), and django includes the annotation
by default... which makes that query slower as well... but testing in
production shows faster results still.

Author: stsewd

readthedocs/projects/models.py

@@ -980,10 +980,10 @@ def conf_dir(self, version=LATEST):
 
     @property
     def has_good_build(self):
-        # Check if there is `_good_build` annotation in the Queryset.
-        # Used for Database optimization.
-        if hasattr(self, "_good_build"):
-            return self._good_build
+        # Check if there is `_has_good_build` annotation in the queryset.
+        # Used for database optimization.
+        if hasattr(self, "_has_good_build"):
+            return self._has_good_build
         return self.builds(manager=INTERNAL).filter(success=True).exists()
 
     def vcs_repo(self, environment, version):

readthedocs/projects/querysets.py

@@ -3,6 +3,8 @@
 from django.conf import settings
 from django.db import models
 from django.db.models import Count
+from django.db.models import Exists
+from django.db.models import OuterRef
 from django.db.models import Prefetch
 from django.db.models import Q
 
@@ -146,7 +148,13 @@ def prefetch_latest_build(self):
             Build.internal.select_related("version").order_by("-date")[:1],
             to_attr=self.model.LATEST_BUILD_CACHE,
         )
-        return self.prefetch_related(latest_build)
+        query = self.prefetch_related(latest_build)
+
+        # Annotate whether the project has a successful build or not,
+        # to avoid N+1 queries when showing the build status.
+        return query.annotate(
+            _has_good_build=Exists(Build.internal.filter(project=OuterRef("pk"), success=True))
+        )
 
     # Aliases
 

readthedocs/rtd_tests/tests/test_project_views.py

@@ -379,7 +379,8 @@ def setUp(self):
         self.project = get(Project, slug="pip", users=[self.user])
 
     def test_dashboard_number_of_queries(self):
-        for i in range(10):
+        # NOTE: create more than 15 projects, as we paginate by 15.
+        for i in range(30):
             project = get(
                 Project,
                 slug=f"project-{i}",
@@ -398,7 +399,7 @@ def test_dashboard_number_of_queries(self):
                     state=BUILD_STATE_FINISHED,
                 )
 
-        with self.assertNumQueries(23):
+        with self.assertNumQueries(12):
             r = self.client.get(reverse(("projects_dashboard")))
         assert r.status_code == 200