Add delete_on_null kwarg

django_restql/fields.py

@@ -53,6 +53,7 @@ def BaseNestedFieldSerializerFactory(
         *args,
         accept_pk=False,
         accept_pk_only=False,
+        delete_on_null=False,
         allow_remove_all=False,
         create_ops=CREATE_OPERATIONS,
         update_ops=UPDATE_OPERATIONS,
@@ -80,6 +81,14 @@ def BaseNestedFieldSerializerFactory(
         "nested fields, ensure the kwarg `many=True` is set."
     )
 
+    assert not (
+        delete_on_null and accept_pk
+    ), "`delete_on_null=True` can not be used if  `accept_pk=True`."
+
+    assert not (
+        delete_on_null and accept_pk_only
+    ), "`delete_on_null=True` can not be used if  `accept_pk_only=True`."
+
     def join_words(words, many='are', single='is'):
         word_list = ["`" + word + "`" for word in words]
 
@@ -248,10 +257,11 @@ def __repr__(self):
 
     class BaseNestedFieldSerializer(serializer_class, BaseNestedField):
 
-        # might be used before `to_internal_value` method is called
-        # so we're creating this property to make sure it's available
+        # These variables might be used before `to_internal_value` method is called
+        # so we're creating them to make sure they're available
         # as long as the class is created
         is_replaceable = accept_pk_only or accept_pk
+        should_delete_on_null = delete_on_null
 
         class Meta(serializer_class.Meta):
             list_serializer_class = BaseNestedFieldListSerializer

django_restql/mixins.py

@@ -806,9 +806,13 @@ def update_writable_foreignkey_related(self, instance, data):
                 context={**self.context, "parent_operation": UPDATE},
             )
             serializer.is_valid(raise_exception=True)
+
+            delete_previous_nested_obj = False
             if values is None:
                 setattr(instance, field, None)
                 needs_save = True
+                if nested_field_serializer.should_delete_on_null and nested_obj is not None:
+                    delete_previous_nested_obj = True
             else:
                 obj = serializer.save()
                 if nested_obj is None:
@@ -818,6 +822,12 @@ def update_writable_foreignkey_related(self, instance, data):
         if needs_save:
             instance.save()
 
+            # We delete the previous nested obj AFTER we are done saving
+            # the instance to avoid accidental deletion of the instance
+            # itself if on_delete=models.CASCADE is used
+            if delete_previous_nested_obj:
+                nested_obj.delete()
+
     def bulk_create_many_to_many_related(self, field, nested_obj, data):
         # Get nested field serializer
         nested_field_serializer = self.restql_writable_nested_fields[field].child

tests/test_views.py

@@ -2123,6 +2123,29 @@ def test_put_on_writable_nested_nullable_foreignkey_related_field_with_empty_str
             },
         )
 
+    def test_put_with_delete_on_null_kwarg(
+        self,
+    ):
+        course = Course.objects.create(name="Test case", code="TS101")
+        student = Student.objects.create(name="Yezy", age=24, course=course)
+
+        url = reverse_lazy("wstudent-detail", args=[student.id])
+        data = {"name": "yezy", "age": 33, "course": None}
+        response = self.client.put(url, data, format="json")
+
+        self.assertEqual(
+            response.data,
+            {
+                "name": "yezy",
+                "age": 33,
+                "course": None,
+                "phone_numbers": [],
+            },
+        )
+
+        # Check if course has been deleted
+        self.assertEqual(Course.objects.filter(id=course.id).count(), 0)
+
     def test_put_with_add_operation(self):
         url = reverse_lazy("wcourse-detail", args=[self.course2.id])
         data = {"name": "Data Structures", "code": "CS410", "books": {"add": [2]}}

tests/testapp/serializers.py

@@ -179,7 +179,10 @@ class Meta:
 
 
 class WritableStudentSerializer(DynamicFieldsMixin, NestedModelSerializer):
-    course = NestedField(WritableCourseSerializer, allow_null=True, required=False)
+    course = NestedField(
+        WritableCourseSerializer,
+        allow_null=True, required=False, delete_on_null=True
+    )
     phone_numbers = NestedField(
         PhoneSerializer, many=True, required=False, allow_remove_all=True
     )