API v3: optimize project listing (#12355)

This is reviving an old optimization that was
added in
363dccb

but it was added for the old footer API, but it works for API v3 as
well.
This used to be 24 queries, now it went down to 16.

Author: stsewd

readthedocs/api/v3/tests/test_projects.py

@@ -33,6 +33,39 @@ def test_projects_list(self):
             self._get_response_dict("projects-list"),
         )
 
+    def test_number_of_queries_projects_list(self):
+        another_project = get(
+            Project,
+            users=[self.me],
+        )
+        superproject = get(
+            Project,
+            users=[self.me],
+        )
+        subproject = get(
+            Project,
+            users=[self.me],
+        )
+        superproject.add_subproject(subproject)
+
+        main_traslation = get(
+            Project,
+            users=[self.me],
+            language="en",
+        )
+        translation = get(
+            Project,
+            users=[self.me],
+            main_language_project=main_traslation,
+            language="es",
+        )
+        url = reverse("projects-list")
+        self.client.credentials(HTTP_AUTHORIZATION=f"Token {self.token.key}")
+        with self.assertNumQueries(16):
+            response = self.client.get(url)
+            assert response.status_code == 200
+            assert len(response.json()["results"]) == 6
+
     @override_settings(ALLOW_PRIVATE_REPOS=True)
     def test_projects_list_privacy_levels_enabled(self):
         self.client.credentials(HTTP_AUTHORIZATION=f"Token {self.token.key}")

readthedocs/api/v3/views.py

@@ -3,6 +3,7 @@
 from django.contrib.contenttypes.models import ContentType
 from django.db.models import Exists
 from django.db.models import OuterRef
+from django.db.models import Prefetch
 from rest_flex_fields import is_expanded
 from rest_flex_fields.views import FlexFieldsMixin
 from rest_framework import status
@@ -39,6 +40,7 @@
 from readthedocs.oauth.models import RemoteRepositoryRelation
 from readthedocs.organizations.models import Organization
 from readthedocs.organizations.models import Team
+from readthedocs.projects.models import Domain
 from readthedocs.projects.models import EnvironmentVariable
 from readthedocs.projects.models import Project
 from readthedocs.projects.models import ProjectRelationship
@@ -177,11 +179,23 @@ def get_queryset(self):
         # This could be a class attribute and managed on the ``ProjectQuerySetMixin`` in
         # case we want to extend the ``prefetch_related`` to other views as
         # well.
-        return queryset.prefetch_related(
-            "related_projects",
-            "domains",
+        return queryset.select_related(
+            "main_language_project",
+        ).prefetch_related(
             "tags",
             "users",
+            # Prefetch superprojects to avoid N+1 queries when serializing the project.
+            Prefetch(
+                "superprojects",
+                ProjectRelationship.objects.all().select_related("parent"),
+                to_attr="_superprojects",
+            ),
+            # Prefetch the canonical domain to avoid N+1 queries when using the resolver.
+            Prefetch(
+                "domains",
+                Domain.objects.filter(canonical=True),
+                to_attr="_canonical_domains",
+            ),
         )
 
     def create(self, request, *args, **kwargs):