From eaafc54ce26eb789e05d7369e997e07a1c3ec751 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?George-Cristian=20B=C3=AErzan?= <g.birzan@mdg.bg>
Date: Fri, 23 Feb 2018 15:53:42 +0200
Subject: [PATCH] #5848 Allow traversing nullable related fields

---
 rest_framework/relations.py    |  2 +-
 tests/test_model_serializer.py | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/rest_framework/relations.py b/rest_framework/relations.py
index c87b9299a..c4e364cf2 100644
--- a/rest_framework/relations.py
+++ b/rest_framework/relations.py
@@ -174,7 +174,7 @@ class RelatedField(Field):
                 pass
 
         # Standard case, return the object instance.
-        return get_attribute(instance, self.source_attrs)
+        return super(RelatedField, self).get_attribute(instance)
 
     def get_choices(self, cutoff=None):
         queryset = self.get_queryset()
diff --git a/tests/test_model_serializer.py b/tests/test_model_serializer.py
index e55afe03e..e4fc8b37f 100644
--- a/tests/test_model_serializer.py
+++ b/tests/test_model_serializer.py
@@ -23,6 +23,8 @@ from django.utils import six
 from rest_framework import serializers
 from rest_framework.compat import postgres_fields, unicode_repr
 
+from .models import NestedForeignKeySource
+
 
 def dedent(blocktext):
     return '\n'.join([line[12:] for line in blocktext.splitlines()[1:-1]])
@@ -1164,6 +1166,25 @@ class Test5004UniqueChoiceField(TestCase):
 
 
 class TestFieldSource(TestCase):
+    def test_traverse_nullable_fk(self):
+        """
+        A dotted source with nullable elements uses default when any item in the chain is None. #5849.
+
+        Similar to model example from test_serializer.py `test_default_for_multiple_dotted_source` method,
+        but using RelatedField, rather than CharField.
+        """
+        class TestSerializer(serializers.ModelSerializer):
+            target = serializers.PrimaryKeyRelatedField(
+                source='target.target', read_only=True, allow_null=True, default=None
+            )
+
+            class Meta:
+                model = NestedForeignKeySource
+                fields = ('target', )
+
+        model = NestedForeignKeySource.objects.create()
+        assert TestSerializer(model).data['target'] is None
+
     def test_named_field_source(self):
         class TestSerializer(serializers.ModelSerializer):