Fixed #28147 -- Fixed loss of assigned parent when saving child after parent.

Thanks Erwin Junge for the initial patch.

AUTHORS

@@ -271,6 +271,7 @@ answer newbie questions, and generally made Django that much better:
     Erik Karulf <erik@karulf.com>
     Erik Romijn <django@solidlinks.nl>
     eriks@win.tue.nl
+    Erwin Junge <erwin@junge.nl>
     Esdras Beleza <linux@esdrasbeleza.com>
     Espen Grindhaug <http://grindhaug.org/>
     Eugene Lazutkin <http://lazutkin.com/blog/>
@@ -744,6 +745,7 @@ answer newbie questions, and generally made Django that much better:
     Robert Wittams
     Rob Golding-Day <rob@golding-day.com>
     Rob Hudson <https://rob.cogit8.org/>
+    Rob Nguyen <tienrobertnguyenn@gmail.com>
     Robin Munn <http://www.geekforgod.com/>
     Rodrigo Pinheiro Marques de Araújo <fenrrir@gmail.com>
     Romain Garrigues <romain.garrigues.cs@gmail.com>

django/db/models/base.py

@@ -684,14 +684,19 @@ class Model(metaclass=ModelBase):
                 # database to raise an IntegrityError if applicable. If
                 # constraints aren't supported by the database, there's the
                 # unavoidable risk of data corruption.
-                if obj and obj.pk is None:
-                    # Remove the object from a related instance cache.
-                    if not field.remote_field.multiple:
-                        field.remote_field.delete_cached_value(obj)
-                    raise ValueError(
-                        "save() prohibited to prevent data loss due to "
-                        "unsaved related object '%s'." % field.name
-                    )
+                if obj:
+                    if obj.pk is None:
+                        # Remove the object from a related instance cache.
+                        if not field.remote_field.multiple:
+                            field.remote_field.delete_cached_value(obj)
+                        raise ValueError(
+                            "save() prohibited to prevent data loss due to "
+                            "unsaved related object '%s'." % field.name
+                        )
+                    elif getattr(self, field.attname) is None:
+                        # Use pk from related object if it has been saved after
+                        # an assignment.
+                        setattr(self, field.attname, obj.pk)
                 # If the relationship's pk/to_field was changed, clear the
                 # cached relationship.
                 if obj and getattr(obj, field.target_field.attname) != getattr(self, field.attname):

django/db/models/fields/related.py

@@ -1031,6 +1031,10 @@ class OneToOneField(ForeignKey):
             setattr(instance, self.name, data)
         else:
             setattr(instance, self.attname, data)
+            # Remote field object must be cleared otherwise Model.save()
+            # will reassign attname using the related object pk.
+            if data is None:
+                setattr(instance, self.name, data)
 
     def _check_unique(self, **kwargs):
         # Override ForeignKey since check isn't applicable here.

tests/many_to_one/models.py

@@ -70,6 +70,10 @@ class Child(models.Model):
     parent = models.ForeignKey(Parent, models.CASCADE)
 
 
+class ChildNullableParent(models.Model):
+    parent = models.ForeignKey(Parent, models.CASCADE, null=True)
+
+
 class ToFieldChild(models.Model):
     parent = models.ForeignKey(Parent, models.CASCADE, to_field='name', related_name='to_field_children')
 

tests/many_to_one/tests.py

@@ -8,8 +8,8 @@ from django.test import TestCase
 from django.utils.translation import gettext_lazy
 
 from .models import (
-    Article, Category, Child, City, District, First, Parent, Record, Relation,
-    Reporter, School, Student, Third, ToFieldChild,
+    Article, Category, Child, ChildNullableParent, City, District, First,
+    Parent, Record, Relation, Reporter, School, Student, Third, ToFieldChild,
 )
 
 
@@ -522,6 +522,14 @@ class ManyToOneTests(TestCase):
         self.assertIsNot(c.parent, p)
         self.assertEqual(c.parent, p)
 
+    def test_save_nullable_fk_after_parent(self):
+        parent = Parent()
+        child = ChildNullableParent(parent=parent)
+        parent.save()
+        child.save()
+        child.refresh_from_db()
+        self.assertEqual(child.parent, parent)
+
     def test_save_nullable_fk_after_parent_with_to_field(self):
         parent = Parent(name='jeff')
         child = ToFieldChild(parent=parent)