Fixes unexpected deletion of related objects.

drf_writable_nested/mixins.py

@@ -333,6 +333,12 @@ def delete_reverse_relations_if_need(self, instance, reverse_relations):
                     # Remove relations from m2m table
                     m2m_manager = getattr(instance, field_source)
                     m2m_manager.remove(*pks_to_delete)
+                elif related_field.many_to_one and related_field.null:
+                    # The instance should not be deleted if the object is foreign-key related.
+                    unlink_foreign_key = {
+                        related_field.name: None,
+                    }
+                    model_class.objects.filter(pk__in=pks_to_delete).update(**unlink_foreign_key)
                 else:
                     model_class.objects.filter(pk__in=pks_to_delete).delete()
 

tests/models.py

@@ -65,15 +65,25 @@ class CustomPK(models.Model):
     )
 
 
-class Message(models.Model):
+class AbstractMessage(models.Model):
     id = models.UUIDField(
         primary_key=True,
         default=uuid.uuid4,
         editable=False
     )
-    profile = models.ForeignKey(Profile, on_delete=models.CASCADE)
     message = models.CharField(max_length=100)
 
+    class Meta:
+        abstract = True
+
+
+class Message(AbstractMessage):
+    profile = models.ForeignKey(Profile, on_delete=models.CASCADE)
+
+
+class PersistentMessage(AbstractMessage):
+    profile = models.ForeignKey(Profile, on_delete=models.CASCADE, null=True)
+
 
 class AnotherProfile(models.Model):
     sites = models.ManyToManyField(Site)

tests/serializers.py

@@ -21,6 +21,13 @@ class Meta:
         fields = ('pk', 'message',)
 
 
+class PersistentMessageSerializer(serializers.ModelSerializer):
+
+    class Meta:
+        model = models.PersistentMessage
+        fields = ('pk', 'message',)
+
+
 class SiteSerializer(serializers.ModelSerializer):
     url = serializers.CharField()
 
@@ -54,6 +61,13 @@ class Meta:
         fields = ('pk', 'sites', 'avatars', 'access_key', 'message_set',)
 
 
+class PersistentProfileSerializer(ProfileSerializer):
+    message_set = PersistentMessageSerializer(many=True)
+
+    class Meta(ProfileSerializer.Meta):
+        pass
+
+
 class UserSerializer(WritableNestedModelSerializer):
     # Reverse OneToOne relation
     profile = ProfileSerializer(required=False, allow_null=True)
@@ -64,6 +78,13 @@ class Meta:
         fields = ('pk', 'profile', 'username', 'user_avatar')
 
 
+class PersistentUserSerializer(WritableNestedModelSerializer):
+    profile = PersistentProfileSerializer(required=False, allow_null=True)
+
+    class Meta(UserSerializer.Meta):
+        pass
+
+
 class CustomSerializer(UserSerializer):
     # Simulate having non-modelfield information on the serializer
     custom_field = serializers.CharField()

tests/test_writable_nested_model_serializer.py

@@ -224,6 +224,49 @@ def test_update(self):
         # Sites shouldn't be deleted either as it is M2M
         self.assertEqual(models.Site.objects.count(), 3)
 
+    def test_update_no_delete_if_null_is_false(self):
+        serializer = serializers.UserSerializer(data=self.get_initial_data())
+        serializer.is_valid(raise_exception=True)
+        user = serializer.save()
+
+        # Check instances count
+        self.assertEqual(models.User.objects.count(), 1)
+        self.assertEqual(models.Message.objects.count(), 3)
+
+        # Update
+        user_pk = user.pk
+        profile_pk = user.profile.pk
+
+        serializer = serializers.PersistentUserSerializer(
+            data={
+                'pk': user_pk,
+                'username': 'new',
+                'profile': {
+                    'pk': profile_pk,
+                    'access_key': None,
+                    'sites': [
+                        {
+                            'url': 'http://new-site.com',
+                        },
+                    ],
+                    'avatars': [
+                    ],
+                    'message_set': [
+                    ],
+                },
+            },
+            instance=user,
+        )
+
+        serializer.is_valid(raise_exception=True)
+        user = serializer.save()
+        user.refresh_from_db()
+
+        # Check instances count
+        self.assertEqual(models.User.objects.count(), 1)
+        self.assertEqual(models.Avatar.objects.count(), 0)
+        self.assertEqual(models.Message.objects.count(), 3)
+
     def test_update_reverse_one_to_one_without_pk(self):
         serializer = serializers.UserSerializer(data=self.get_initial_data())
         serializer.is_valid(raise_exception=True)