Avoid error with related urls when retrieving relationship

CHANGELOG.md

@@ -24,6 +24,7 @@ any parts of the framework not mentioned in the documentation should generally b
 
 * 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))
+* Avoid error with related urls when retrieving relationship which is referenced as `ForeignKey` on parent
 
 
 ## [2.6.0] - 2018-09-20

example/serializers.py

@@ -225,6 +225,7 @@ class AuthorSerializer(serializers.ModelSerializer):
     }
     related_serializers = {
         'bio': 'example.serializers.AuthorBioSerializer',
+        'type': 'example.serializers.AuthorTypeSerializer',
         'entries': 'example.serializers.EntrySerializer',
         'first_entry': 'example.serializers.EntrySerializer'
     }

example/tests/test_views.py

@@ -325,11 +325,11 @@ def test_get_serializer_comes_from_included_serializers(self):
         view.serializer_class.related_serializers = related_serializers
 
     def test_get_serializer_class_raises_error(self):
-        kwargs = {'pk': self.author.id, 'related_field': 'type'}
+        kwargs = {'pk': self.author.id, 'related_field': 'unknown'}
         view = self._get_view(kwargs)
         self.assertRaises(NotFound, view.get_serializer_class)
 
-    def test_retrieve_related_single(self):
+    def test_retrieve_related_single_reverse_lookup(self):
         url = reverse('author-related', kwargs={'pk': self.author.pk, 'related_field': 'bio'})
         resp = self.client.get(url)
         expected = {
@@ -345,6 +345,20 @@ def test_retrieve_related_single(self):
         self.assertEqual(resp.status_code, 200)
         self.assertEqual(resp.json(), expected)
 
+    def test_retrieve_related_single(self):
+        url = reverse('author-related', kwargs={'pk': self.author.type.pk, 'related_field': 'type'})
+        resp = self.client.get(url)
+        expected = {
+            'data': {
+                'type': 'authorTypes', 'id': str(self.author.type.id),
+                'attributes': {
+                    'name': str(self.author.type.name)
+                },
+            }
+        }
+        self.assertEqual(resp.status_code, 200)
+        self.assertEqual(resp.json(), expected)
+
     def test_retrieve_related_many(self):
         entry = EntryFactory(authors=self.author)
         url = reverse('author-related', kwargs={'pk': self.author.pk, 'related_field': 'entries'})

rest_framework_json_api/views.py

@@ -14,6 +14,8 @@
 from django.utils.module_loading import import_string as import_class_from_dotted_path
 from rest_framework import generics, viewsets
 from rest_framework.exceptions import MethodNotAllowed, NotFound
+from rest_framework.fields import get_attribute
+from rest_framework.relations import PKOnlyObject
 from rest_framework.response import Response
 from rest_framework.reverse import reverse
 from rest_framework.serializers import Serializer
@@ -164,7 +166,11 @@ def get_related_instance(self):
         field = parent_serializer.fields.get(field_name, None)
 
         if field is not None:
-            return field.get_attribute(parent_obj)
+            instance = field.get_attribute(parent_obj)
+            if isinstance(instance, PKOnlyObject):
+                # need whole object
+                instance = get_attribute(parent_obj, field.source_attrs)
+            return instance
         else:
             try:
                 return getattr(parent_obj, field_name)