[1.7.x] Refs #25693 -- Added a regression test for `to_attr` validation on forward m2m.

Backport of cc8c02fa0f from master

django/db/models/query.py

@@ -10,7 +10,7 @@ from django.conf import settings
 from django.core import exceptions
 from django.db import connections, router, transaction, IntegrityError
 from django.db.models.constants import LOOKUP_SEP
-from django.db.models.fields import AutoField, Empty
+from django.db.models.fields import AutoField, Empty, FieldDoesNotExist
 from django.db.models.query_utils import (Q, select_related_descend,
     deferred_class_factory, InvalidQuery)
 from django.db.models.deletion import Collector
@@ -1912,10 +1912,20 @@ def prefetch_one_level(instances, prefetcher, lookup, level):
         # We assume that objects retrieved are homogeneous (which is the premise
         # of prefetch_related), so what applies to first object applies to all.
         model = instances[0].__class__
-        for related_m2m in model._meta.get_all_related_many_to_many_objects():
-            if related_m2m.get_accessor_name() == to_attr:
-                msg = 'to_attr={} conflicts with a field on the {} model.'
-                raise ValueError(msg.format(to_attr, model.__name__))
+        opts = model._meta
+        conflicts = False
+        try:
+            opts.get_field(to_attr)
+        except FieldDoesNotExist:
+            for related_m2m in opts.get_all_related_many_to_many_objects():
+                if related_m2m.get_accessor_name() == to_attr:
+                    conflicts = True
+                    break
+        else:
+            conflicts = True
+        if conflicts:
+            msg = 'to_attr={} conflicts with a field on the {} model.'
+            raise ValueError(msg.format(to_attr, model.__name__))
 
     for obj in instances:
         instance_attr_val = instance_attr(obj)

tests/prefetch_related/tests.py

@@ -221,7 +221,18 @@ class PrefetchRelatedTests(TestCase):
         self.assertTrue('prefetch_related' in str(cm.exception))
         self.assertTrue("name" in str(cm.exception))
 
-    def test_m2m_shadow(self):
+    def test_forward_m2m_to_attr_conflict(self):
+        msg = 'to_attr=authors conflicts with a field on the Book model.'
+        authors = Author.objects.all()
+        with self.assertRaisesMessage(ValueError, msg):
+            list(Book.objects.prefetch_related(
+                Prefetch('authors', queryset=authors, to_attr='authors'),
+            ))
+        # Without the ValueError, an author was deleted due to the implicit
+        # save of the relation assignment.
+        self.assertEqual(self.book1.authors.count(), 3)
+
+    def test_reverse_m2m_to_attr_conflict(self):
         msg = 'to_attr=books conflicts with a field on the Author model.'
         poems = Book.objects.filter(title='Poems')
         with self.assertRaisesMessage(ValueError, msg):