Refs #28586 -- Copied fetch modes to related objects.

This change ensures that behavior and performance remain consistent when
traversing relationships.

tests/prefetch_related/tests.py

@@ -3,7 +3,13 @@ from unittest import mock
 from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import ObjectDoesNotExist
 from django.db import NotSupportedError, connection
-from django.db.models import F, Prefetch, QuerySet, prefetch_related_objects
+from django.db.models import (
+    FETCH_PEERS,
+    F,
+    Prefetch,
+    QuerySet,
+    prefetch_related_objects,
+)
 from django.db.models.fetch_modes import RAISE
 from django.db.models.query import get_prefetcher
 from django.db.models.sql import Query
@@ -108,6 +114,28 @@ class PrefetchRelatedTests(TestDataMixin, TestCase):
         normal_books = [a.first_book for a in Author.objects.all()]
         self.assertEqual(books, normal_books)
 
+    def test_fetch_mode_copied_fetching_one(self):
+        author = (
+            Author.objects.fetch_mode(FETCH_PEERS)
+            .prefetch_related("first_book")
+            .get(pk=self.author1.pk)
+        )
+        self.assertEqual(author._state.fetch_mode, FETCH_PEERS)
+        self.assertEqual(
+            author.first_book._state.fetch_mode,
+            FETCH_PEERS,
+        )
+
+    def test_fetch_mode_copied_fetching_many(self):
+        authors = list(
+            Author.objects.fetch_mode(FETCH_PEERS).prefetch_related("first_book")
+        )
+        self.assertEqual(authors[0]._state.fetch_mode, FETCH_PEERS)
+        self.assertEqual(
+            authors[0].first_book._state.fetch_mode,
+            FETCH_PEERS,
+        )
+
     def test_fetch_mode_raise(self):
         authors = list(Author.objects.fetch_mode(RAISE).prefetch_related("first_book"))
         authors[0].first_book  # No exception, already loaded