Fixed #33322 -- Fixed loss of assigned related object when saving relation with bulk_update().

django/db/models/base.py

@@ -931,11 +931,13 @@ class Model(metaclass=ModelBase):
             using=using, raw=raw,
         )
 
-    def _prepare_related_fields_for_save(self, operation_name):
+    def _prepare_related_fields_for_save(self, operation_name, fields=None):
         # Ensure that a model instance without a PK hasn't been assigned to
         # a ForeignKey or OneToOneField on this model. If the field is
         # nullable, allowing the save would result in silent data loss.
         for field in self._meta.concrete_fields:
+            if fields and field not in fields:
+                continue
             # If the related field isn't cached, then an instance hasn't been
             # assigned and there's no need to worry about this check.
             if field.is_relation and field.is_cached(self):

django/db/models/query.py

@@ -549,6 +549,8 @@ class QuerySet:
             raise ValueError('bulk_update() cannot be used with primary key fields.')
         if not objs:
             return 0
+        for obj in objs:
+            obj._prepare_related_fields_for_save(operation_name='bulk_update', fields=fields)
         # PK is used twice in the resulting update query, once in the filter
         # and once in the WHEN. Each field will also have one CAST.
         connection = connections[self.db]

tests/queries/test_bulk_update.py

@@ -7,7 +7,8 @@ from django.test import TestCase, skipUnlessDBFeature
 
 from .models import (
     Article, CustomDbColumn, CustomPk, Detail, Individual, JSONFieldNullable,
-    Member, Note, Number, Order, Paragraph, SpecialCategory, Tag, Valid,
+    Member, Note, Number, Order, Paragraph, RelatedObject, SingleObject,
+    SpecialCategory, Tag, Valid,
 )
 
 
@@ -250,3 +251,32 @@ class BulkUpdateTests(TestCase):
             obj.json_field = {'c': obj.json_field['a'] + 1}
         JSONFieldNullable.objects.bulk_update(objs, ['json_field'])
         self.assertCountEqual(JSONFieldNullable.objects.filter(json_field__has_key='c'), objs)
+
+    def test_nullable_fk_after_related_save(self):
+        parent = RelatedObject.objects.create()
+        child = SingleObject()
+        parent.single = child
+        parent.single.save()
+        RelatedObject.objects.bulk_update([parent], fields=['single'])
+        self.assertEqual(parent.single_id, parent.single.pk)
+        parent.refresh_from_db()
+        self.assertEqual(parent.single, child)
+
+    def test_unsaved_parent(self):
+        parent = RelatedObject.objects.create()
+        parent.single = SingleObject()
+        msg = (
+            "bulk_update() prohibited to prevent data loss due to unsaved "
+            "related object 'single'."
+        )
+        with self.assertRaisesMessage(ValueError, msg):
+            RelatedObject.objects.bulk_update([parent], fields=['single'])
+
+    def test_unspecified_unsaved_parent(self):
+        parent = RelatedObject.objects.create()
+        parent.single = SingleObject()
+        parent.f = 42
+        RelatedObject.objects.bulk_update([parent], fields=['f'])
+        parent.refresh_from_db()
+        self.assertEqual(parent.f, 42)
+        self.assertIsNone(parent.single)