FETCH_PEERS does not apply to instances loaded by select_related()

[Mykhailo Havelia]

Description

Combining select_related() with FETCH_PEERS silently disables peer batching one level below the join, turning an intended optimization into an N+1 and making the query count *worse* than not calling select_related() at all.

Given Species -> Genus -> Family:

# 3 queries: species, batch of genus, batch of family
for species in Species.objects.fetch_mode(FETCH_PEERS):
    species.genus.family.name

# 4 queries: the join, then ONE QUERY PER FAMILY
for species in Species.objects.select_related("genus").fetch_mode(FETCH_PEERS):
    species.genus.family.name

The second form emits:

SELECT ... FROM species INNER JOIN genus ON (species.genus_id = genus.id);
SELECT ... FROM family WHERE family.id = 1 LIMIT 21;
SELECT ... FROM family WHERE family.id = 2 LIMIT 21;
SELECT ... FROM family WHERE family.id = 3 LIMIT 21;

Was it intended to be this way? If so, I think it’s worth mentioning in the documentation.

[Sarah Boyce]

Thank you for the report! Replicated and to me, this feels like a bug but I will let folks chime in as to whether this is expected behavior

tests/select_related/tests.py

@@ -281 +281 @@
             FETCH_PEERS,
         )
 
+    def test_fetch_peers_for_select_related_objects(self):
+        species = Species.objects.select_related("genus__family").fetch_mode(
+            FETCH_PEERS
+        )
+        with self.assertNumQueries(2):
+            orders = [obj.genus.family.order.name for obj in species]
+            self.assertEqual(
+                orders,
+                ["Diptera", "Primates", "Fabales", "Agaricales"],
+            )
+
 
 class SelectRelatedValidationTests(SimpleTestCase):

Refs #28586

[Sarah Boyce <42296566+sarahboyce@…>]

In 6ade625:

Fixed #37344 -- Enabled FETCH_PEERS batching for instances loaded by select_related.

Thank you to Mykhailo Havelia for the report.

[Sarah Boyce <42296566+sarahboyce@…>]

In 64c6e09:

[6.1.x] Fixed #37344 -- Enabled FETCH_PEERS batching for instances loaded by select_related.

Thank you to Mykhailo Havelia for the report.

Backport of 6ade6258480fba84a7e895b4b1e1716dfc954778 from main.