Dashboard: don't prefetch latest build

readthedocs/projects/models.py

@@ -1,6 +1,7 @@
 """Project models."""
 
 import fnmatch
+from functools import lru_cache
 import hashlib
 import hmac
 import os
@@ -645,9 +646,6 @@ class Project(models.Model):
         blank=True,
     )
 
-    # Property used for storing the latest build for a project when prefetching
-    LATEST_BUILD_CACHE = "_latest_build"
-
     class Meta:
         ordering = ("slug",)
         verbose_name = _("project")
@@ -1100,23 +1098,17 @@ def full_find(self, filename, version):
                 matches.append(os.path.join(root, match))
         return matches
 
+    @lru_cache(maxsize=1)
     def get_latest_build(self, finished=True):
         """
         Get latest build for project.
 
         :param finished: Return only builds that are in a finished state
         """
-        # Check if there is `_latest_build` attribute in the Queryset.
-        # Used for Database optimization.
-        if hasattr(self, self.LATEST_BUILD_CACHE):
-            if self._latest_build:
-                return self._latest_build[0]
-            return None
-
-        kwargs = {"type": "html"}
+        kwargs = {}
         if finished:
             kwargs["state"] = "finished"
-        return self.builds(manager=INTERNAL).filter(**kwargs).first()
+        return self.builds(manager=INTERNAL).filter(**kwargs).select_related("version").first()
 
     def active_versions(self):
         from readthedocs.builds.models import Version

readthedocs/projects/querysets.py

@@ -5,7 +5,6 @@
 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
 
 from readthedocs.core.permissions import AdminPermission
@@ -134,25 +133,26 @@ def max_concurrent_builds(self, project):
 
     def prefetch_latest_build(self):
         """
-        Prefetch "latest build" for each project.
+        Prefetch and annotate to avoid N+1 queries.
 
         .. note::
 
             This should come after any filtering.
         """
         from readthedocs.builds.models import Build
 
-        # Prefetch the latest build for each project.
-        latest_build = Prefetch(
-            "builds",
-            Build.internal.select_related("version").order_by("-date")[:1],
-            to_attr=self.model.LATEST_BUILD_CACHE,
-        )
-        query = self.prefetch_related(latest_build)
+        # NOTE: prefetching the latest build will perform worse than just
+        # accessing the latest build for each project.
+        # While prefetching reduces the number of queries,
+        # the query used to fetch the latest build can be quite expensive,
+        # specially in projects with lots of builds.
+        # Not prefetching here is fine, as this query is paginated by 15
+        # items per page, so it will generate at most 15 queries.
 
+        # This annotation performs fine in all cases.
         # Annotate whether the project has a successful build or not,
         # to avoid N+1 queries when showing the build status.
-        return query.annotate(
+        return self.annotate(
             _has_good_build=Exists(Build.internal.filter(project=OuterRef("pk"), success=True))
         )
 

readthedocs/rtd_tests/tests/test_project_views.py

@@ -399,7 +399,7 @@ def test_dashboard_number_of_queries(self):
                     state=BUILD_STATE_FINISHED,
                 )
 
-        with self.assertNumQueries(12):
+        with self.assertNumQueries(27):
             r = self.client.get(reverse(("projects_dashboard")))
         assert r.status_code == 200