Avoid patch on RelationshipView deleting relationship instance when constraint would allow null

CHANGELOG.md

@@ -23,6 +23,7 @@ any parts of the framework not mentioned in the documentation should generally b
 ### Fixed
 
 * Pass context from `PolymorphicModelSerializer` to child serializers to support fields which require a `request` context such as `url`.
+* Avoid patch on `RelationshipView` deleting relationship instance when constraint would allow null ([#242](https://github.com/django-json-api/django-rest-framework-json-api/issues/242))
 
 
 ## [2.6.0] - 2018-09-20

example/tests/test_views.py

@@ -11,7 +11,7 @@
 
 from . import TestBase
 from .. import views
-from example.factories import AuthorFactory, EntryFactory
+from example.factories import AuthorFactory, CommentFactory, EntryFactory
 from example.models import Author, Blog, Comment, Entry
 from example.serializers import AuthorBioSerializer, AuthorTypeSerializer, EntrySerializer
 from example.views import AuthorViewSet
@@ -229,6 +229,50 @@ def test_delete_to_many_relationship_with_change(self):
         response = self.client.delete(url, data=request_data)
         assert response.status_code == 200, response.content.decode()
 
+    def test_new_comment_data_patch_to_many_relationship(self):
+        entry = EntryFactory(blog=self.blog, authors=(self.author,))
+        comment = CommentFactory(entry=entry)
+
+        url = '/authors/{}/relationships/comment_set'.format(self.author.id)
+        request_data = {
+            'data': [{'type': format_resource_type('Comment'), 'id': str(comment.id)}, ]
+        }
+        previous_response = {
+            'data': [
+                {'type': 'comments',
+                 'id': str(self.second_comment.id)
+                 }
+            ],
+            'links': {
+                'self': 'http://testserver/authors/{}/relationships/comment_set'.format(
+                    self.author.id
+                )
+            }
+        }
+
+        response = self.client.get(url)
+        assert response.status_code == 200
+        assert response.json() == previous_response
+
+        new_patched_response = {
+            'data': [
+                {'type': 'comments',
+                 'id': str(comment.id)
+                 }
+            ],
+            'links': {
+                'self': 'http://testserver/authors/{}/relationships/comment_set'.format(
+                    self.author.id
+                )
+            }
+        }
+
+        response = self.client.patch(url, data=request_data)
+        assert response.status_code == 200
+        assert response.json() == new_patched_response
+
+        assert Comment.objects.filter(id=self.second_comment.id).exists()
+
 
 class TestRelatedMixin(APITestCase):
 

rest_framework_json_api/views.py

@@ -251,6 +251,18 @@ def get(self, request, *args, **kwargs):
         serializer_instance = self._instantiate_serializer(related_instance)
         return Response(serializer_instance.data)
 
+    def remove_relationships(self, instance_manager, field):
+        field_object = getattr(instance_manager, field)
+
+        if field_object.null:
+            for obj in instance_manager.all():
+                setattr(obj, field_object.name, None)
+                obj.save()
+        else:
+            instance_manager.all().delete()
+
+        return instance_manager
+
     def patch(self, request, *args, **kwargs):
         parent_obj = self.get_object()
         related_instance_or_manager = self.get_related_instance()
@@ -261,7 +273,16 @@ def patch(self, request, *args, **kwargs):
                 data=request.data, model_class=related_model_class, many=True
             )
             serializer.is_valid(raise_exception=True)
-            related_instance_or_manager.all().delete()
+
+            # for to one
+            if hasattr(related_instance_or_manager, "field"):
+                related_instance_or_manager = self.remove_relationships(
+                    instance_manager=related_instance_or_manager, field="field")
+            # for to many
+            else:
+                related_instance_or_manager = self.remove_relationships(
+                    instance_manager=related_instance_or_manager, field="target_field")
+
             # have to set bulk to False since data isn't saved yet
             class_name = related_instance_or_manager.__class__.__name__
             if class_name != 'ManyRelatedManager':