Fixed instance being overwritten in pk-only optimization try/except block (#5747)

rest_framework/relations.py

@@ -163,8 +163,8 @@ class RelatedField(Field):
         if self.use_pk_only_optimization() and self.source_attrs:
             # Optimized case, return a mock object only containing the pk attribute.
             try:
-                instance = get_attribute(instance, self.source_attrs[:-1])
+                attribute_instance = get_attribute(instance, self.source_attrs[:-1])
-                value = instance.serializable_value(self.source_attrs[-1])
+                value = attribute_instance.serializable_value(self.source_attrs[-1])
                 if is_simple_callable(value):
                     # Handle edge case where the relationship `source` argument
                     # points to a `get_relationship()` method on the model

tests/test_relations.py

@@ -3,7 +3,7 @@ import uuid
 import pytest
 from _pytest.monkeypatch import MonkeyPatch
 from django.conf.urls import url
-from django.core.exceptions import ImproperlyConfigured
+from django.core.exceptions import ImproperlyConfigured, ObjectDoesNotExist
 from django.test import override_settings
 from django.utils.datastructures import MultiValueDict
 
@@ -167,6 +167,22 @@ class TestHyperlinkedRelatedField(APISimpleTestCase):
         representation = self.field.to_representation(MockObject(pk=''))
         assert representation is None
 
+    def test_serialize_empty_relationship_attribute(self):
+        class TestSerializer(serializers.Serializer):
+            via_unreachable = serializers.HyperlinkedRelatedField(
+                source='does_not_exist.unreachable',
+                view_name='example',
+                read_only=True,
+            )
+
+        class TestSerializable:
+            @property
+            def does_not_exist(self):
+                raise ObjectDoesNotExist
+
+        serializer = TestSerializer(TestSerializable())
+        assert serializer.data == {'via_unreachable': None}
+
     def test_hyperlinked_related_lookup_exists(self):
         instance = self.field.to_internal_value('http://example.org/example/foobar/')
         assert instance is self.queryset.items[0]