Fixed #35442 -- Prevented N+1 queries in RelatedManager with only().

Co-authored-by: Simon Charette <charette.s@gmail.com>

2 files changed, 26 insertions, 14 deletions

diff --git a/django/db/models/query.py b/django/db/models/query.py
index 73d6717bce..5649a83428 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -114,16 +114,15 @@ class ModelIterable(BaseIterable):
             (
                 field,
                 related_objs,
-                operator.attrgetter(
-                    *[
-                        (
-                            field.attname
-                            if from_field == "self"
-                            else queryset.model._meta.get_field(from_field).attname
-                        )
-                        for from_field in field.from_fields
-                    ]
-                ),
+                attnames := [
+                    (
+                        field.attname
+                        if from_field == "self"
+                        else queryset.model._meta.get_field(from_field).attname
+                    )
+                    for from_field in field.from_fields
+                ],
+                operator.attrgetter(*attnames),
             )
             for field, related_objs in queryset._known_related_objects.items()
         ]
@@ -145,10 +144,14 @@ class ModelIterable(BaseIterable):
                     setattr(obj, attr_name, row[col_pos])
 
             # Add the known related objects to the model.
-            for field, rel_objs, rel_getter in known_related_objects:
+            for field, rel_objs, rel_attnames, rel_getter in known_related_objects:
                 # Avoid overwriting objects loaded by, e.g., select_related().
                 if field.is_cached(obj):
                     continue
+                # Avoid fetching potentially deferred attributes that would
+                # result in unexpected queries.
+                if any(attname not in obj.__dict__ for attname in rel_attnames):
+                    continue
                 rel_obj_id = rel_getter(obj)
                 try:
                     rel_obj = rel_objs[rel_obj_id]
diff --git a/tests/defer/tests.py b/tests/defer/tests.py
index 29c63c566a..1a5f333fa1 100644
--- a/tests/defer/tests.py
+++ b/tests/defer/tests.py
@@ -49,9 +49,9 @@ class DeferTests(AssertionMixin, TestCase):
         # of them except the model's primary key see #15494
         self.assert_delayed(qs.only("pk")[0], 3)
         # You can use 'pk' with reverse foreign key lookups.
-        # The related_id is always set even if it's not fetched from the DB,
-        # so pk and related_id are not deferred.
-        self.assert_delayed(self.s1.primary_set.only("pk")[0], 2)
+        # The related_id is not set if it's not fetched from the DB,
+        # so pk is not deferred, but related_id is.
+        self.assert_delayed(self.s1.primary_set.only("pk")[0], 3)
 
     def test_defer_only_chaining(self):
         qs = Primary.objects.all()
@@ -84,6 +84,15 @@ class DeferTests(AssertionMixin, TestCase):
         with self.assertRaisesMessage(TypeError, msg):
             Primary.objects.only(None)
 
+    def test_only_related_manager_optimization(self):
+        s = Secondary.objects.create(first="one", second="two")
+        Primary.objects.bulk_create(
+            [Primary(name="p1", value="v1", related=s) for _ in range(5)]
+        )
+        with self.assertNumQueries(1):
+            for p in s.primary_set.only("pk"):
+                _ = p.pk
+
     def test_defer_extra(self):
         qs = Primary.objects.all()
         self.assert_delayed(qs.defer("name").extra(select={"a": 1})[0], 1)