Prefetch build and project on version list (#11616)

This optimizes the version listing view and removes redundant queries.

- Requires readthedocs/ext-theme#493
- Fixes readthedocs/ext-theme#463

readthedocs/builds/models.py

@@ -207,6 +207,9 @@ class Meta:
         unique_together = [("project", "slug")]
         ordering = ["-verbose_name"]
 
+    # Property used for prefetching version related fields
+    LATEST_BUILD_CACHE = "_latest_build"
+
     def __str__(self):
         return self.verbose_name
 
@@ -291,6 +294,19 @@ def vcs_url(self):
 
     @property
     def last_build(self):
+        # TODO deprecated in favor of `latest_build`, which matches naming on
+        # the Project model
+        return self.latest_build
+
+    @property
+    def latest_build(self):
+        # Check if there is `_latest_build` prefetch in the Queryset.
+        # Used for database optimization.
+        if hasattr(self, self.LATEST_BUILD_CACHE):
+            if latest_build := getattr(self, self.LATEST_BUILD_CACHE):
+                return latest_build[0]
+            return None
+
         return self.builds.order_by("-date").first()
 
     @property

readthedocs/builds/querysets.py

@@ -3,7 +3,7 @@
 
 import structlog
 from django.db import models
-from django.db.models import Q
+from django.db.models import OuterRef, Prefetch, Q, Subquery
 from django.utils import timezone
 
 from readthedocs.builds.constants import (
@@ -141,6 +141,30 @@ def for_reindex(self):
             .distinct()
         )
 
+    def prefetch_subquery(self):
+        """
+        Prefetch related objects via subquery for each version.
+
+        .. note::
+
+            This should come after any filtering.
+        """
+        from readthedocs.builds.models import Build
+
+        # Prefetch the latest build for each project.
+        subquery_builds = Subquery(
+            Build.internal.filter(version=OuterRef("version_id"))
+            .order_by("-date")
+            .values_list("id", flat=True)[:1]
+        )
+        prefetch_builds = Prefetch(
+            "builds",
+            Build.internal.filter(pk__in=subquery_builds),
+            to_attr=self.model.LATEST_BUILD_CACHE,
+        )
+
+        return self.prefetch_related(prefetch_builds)
+
 
 class VersionQuerySet(SettingsOverrideObject):
     _default_class = VersionQuerySetBase

readthedocs/projects/views/public.py

@@ -123,7 +123,11 @@ def get_context_data(self, **kwargs):
                 queryset=versions,
                 project=project,
             )
-            versions = self.get_filtered_queryset()
+            versions = (
+                self.get_filtered_queryset()
+                .prefetch_related("project")
+                .prefetch_subquery()
+            )
         context["versions"] = versions
 
         protocol = "http"