Do not skip empty one-to-one relationships (#526)

Author: sliverc

CHANGELOG.md

@@ -26,6 +26,7 @@ any parts of the framework not mentioned in the documentation should generally b
 * 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))
 * Avoid error with related urls when retrieving relationship which is referenced as `ForeignKey` on parent
 * Do not render `write_only` relations
+* Do not skip empty one-to-one relationships
 
 
 ## [2.6.0] - 2018-09-20

example/tests/unit/test_renderers.py

@@ -3,7 +3,7 @@
 from rest_framework_json_api import serializers, views
 from rest_framework_json_api.renderers import JSONRenderer
 
-from example.models import Comment, Entry
+from example.models import Author, Comment, Entry
 
 
 # serializers
@@ -45,8 +45,8 @@ class ReadOnlyDummyTestViewSet(views.ReadOnlyModelViewSet):
     serializer_class = DummyTestSerializer
 
 
-def render_dummy_test_serialized_view(view_class):
-    serializer = view_class.serializer_class(instance=Entry())
+def render_dummy_test_serialized_view(view_class, instance):
+    serializer = view_class.serializer_class(instance=instance)
     renderer = JSONRenderer()
     return renderer.render(
         serializer.data,
@@ -58,22 +58,22 @@ def test_simple_reverse_relation_included_renderer():
     Test renderer when a single reverse fk relation is passed.
     '''
     rendered = render_dummy_test_serialized_view(
-        DummyTestViewSet)
+        DummyTestViewSet, Entry())
 
     assert rendered
 
 
 def test_simple_reverse_relation_included_read_only_viewset():
     rendered = render_dummy_test_serialized_view(
-        ReadOnlyDummyTestViewSet)
+        ReadOnlyDummyTestViewSet, Entry())
 
     assert rendered
 
 
 def test_render_format_field_names(settings):
     """Test that json field is kept untouched."""
     settings.JSON_API_FORMAT_FIELD_NAMES = 'dasherize'
-    rendered = render_dummy_test_serialized_view(DummyTestViewSet)
+    rendered = render_dummy_test_serialized_view(DummyTestViewSet, Entry())
 
     result = json.loads(rendered.decode())
     assert result['data']['attributes']['json-field'] == {'JsonKey': 'JsonValue'}
@@ -83,17 +83,15 @@ def test_render_format_keys(settings):
     """Test that json field value keys are formated."""
     delattr(settings, 'JSON_API_FORMAT_FILED_NAMES')
     settings.JSON_API_FORMAT_KEYS = 'dasherize'
-    rendered = render_dummy_test_serialized_view(DummyTestViewSet)
+    rendered = render_dummy_test_serialized_view(DummyTestViewSet, Entry())
 
     result = json.loads(rendered.decode())
     assert result['data']['attributes']['json-field'] == {'json-key': 'JsonValue'}
 
 
-def test_writeonly_not_in_response(settings):
+def test_writeonly_not_in_response():
     """Test that writeonly fields are not shown in list response"""
 
-    settings.JSON_API_FORMAT_FIELD_NAMES = 'dasherize'
-
     class WriteonlyTestSerializer(serializers.ModelSerializer):
         '''Serializer for testing the absence of write_only fields'''
         comments = serializers.ResourceRelatedField(
@@ -112,8 +110,27 @@ class WriteOnlyDummyTestViewSet(views.ReadOnlyModelViewSet):
         queryset = Entry.objects.all()
         serializer_class = WriteonlyTestSerializer
 
-    rendered = render_dummy_test_serialized_view(WriteOnlyDummyTestViewSet)
+    rendered = render_dummy_test_serialized_view(WriteOnlyDummyTestViewSet, Entry())
     result = json.loads(rendered.decode())
 
     assert 'rating' not in result['data']['attributes']
     assert 'relationships' not in result['data']
+
+
+def test_render_empty_relationship_reverse_lookup():
+    """Test that empty relationships are rendered as None."""
+
+    class EmptyRelationshipSerializer(serializers.ModelSerializer):
+        class Meta:
+            model = Author
+            fields = ('bio', )
+
+    class EmptyRelationshipViewSet(views.ReadOnlyModelViewSet):
+        queryset = Author.objects.all()
+        serializer_class = EmptyRelationshipSerializer
+
+    rendered = render_dummy_test_serialized_view(EmptyRelationshipViewSet, Author())
+    result = json.loads(rendered.decode())
+    assert 'relationships' in result['data']
+    assert 'bio' in result['data']['relationships']
+    assert result['data']['relationships']['bio'] == {'data': None}

rest_framework_json_api/renderers.py

@@ -150,13 +150,6 @@ def extract_relationships(cls, fields, resource, resource_instance):
                 data.update({field_name: relation_data})
 
             if isinstance(field, (ResourceRelatedField, )):
-                relation_instance_id = getattr(resource_instance, source + "_id", None)
-                if not relation_instance_id:
-                    resolved, relation_instance = utils.get_relation_instance(resource_instance,
-                                                                              source, field.parent)
-                    if not resolved:
-                        continue
-
                 if not isinstance(field, SkipDataMixin):
                     relation_data.update({'data': resource.get(field_name)})