Fix None UUID ForeignKey serialization

docs/topics/release-notes.md

@@ -45,6 +45,7 @@ You can determine your currently installed version using `pip freeze`:
 **Unreleased**
 
 * Dropped support for EOL Django 1.7 ([#3933][gh3933])
+* Fixed null foreign keys targeting UUIDField primary keys. ([#3936][gh3936])
 
 ### 3.3.2
 

rest_framework/serializers.py

@@ -464,9 +464,13 @@ def to_representation(self, instance):
             except SkipField:
                 continue
 
-            if attribute is None:
-                # We skip `to_representation` for `None` values so that
-                # fields do not have to explicitly deal with that case.
+            # We skip `to_representation` for `None` values so that fields do
+            # not have to explicitly deal with that case.
+            #
+            # For related fields with `use_pk_only_optimization` we need to
+            # resolve the pk value.
+            check_for_none = attribute.pk if isinstance(attribute, PKOnlyObject) else attribute
+            if check_for_none is None:
                 ret[field.field_name] = None
             else:
                 ret[field.field_name] = field.to_representation(attribute)

tests/models.py

@@ -1,5 +1,7 @@
 from __future__ import unicode_literals
 
+import uuid
+
 from django.db import models
 from django.utils.translation import ugettext_lazy as _
 
@@ -46,6 +48,11 @@ class ForeignKeyTarget(RESTFrameworkModel):
     name = models.CharField(max_length=100)
 
 
+class UUIDForeignKeyTarget(RESTFrameworkModel):
+    uuid = models.UUIDField(primary_key=True, default=uuid.uuid4)
+    name = models.CharField(max_length=100)
+
+
 class ForeignKeySource(RESTFrameworkModel):
     name = models.CharField(max_length=100)
     target = models.ForeignKey(ForeignKeyTarget, related_name='sources',
@@ -62,6 +69,14 @@ class NullableForeignKeySource(RESTFrameworkModel):
                                on_delete=models.CASCADE)
 
 
+class NullableUUIDForeignKeySource(RESTFrameworkModel):
+    name = models.CharField(max_length=100)
+    target = models.ForeignKey(ForeignKeyTarget, null=True, blank=True,
+                               related_name='nullable_sources',
+                               verbose_name='Optional target object',
+                               on_delete=models.CASCADE)
+
+
 # OneToOne
 class OneToOneTarget(RESTFrameworkModel):
     name = models.CharField(max_length=100)

tests/test_relations_pk.py

@@ -6,7 +6,8 @@
 from rest_framework import serializers
 from tests.models import (
     ForeignKeySource, ForeignKeyTarget, ManyToManySource, ManyToManyTarget,
-    NullableForeignKeySource, NullableOneToOneSource, OneToOneTarget
+    NullableForeignKeySource, NullableOneToOneSource,
+    NullableUUIDForeignKeySource, OneToOneTarget, UUIDForeignKeyTarget
 )
 
 
@@ -43,6 +44,18 @@ class Meta:
         fields = ('id', 'name', 'target')
 
 
+# Nullable UUIDForeignKey
+class NullableUUIDForeignKeySourceSerializer(serializers.ModelSerializer):
+    target = serializers.PrimaryKeyRelatedField(
+        pk_field=serializers.UUIDField(),
+        queryset=UUIDForeignKeyTarget.objects.all(),
+        allow_null=True)
+
+    class Meta:
+        model = NullableUUIDForeignKeySource
+        fields = ('id', 'name', 'target')
+
+
 # Nullable OneToOne
 class NullableOneToOneTargetSerializer(serializers.ModelSerializer):
     class Meta:
@@ -432,6 +445,17 @@ def test_foreign_key_update_with_valid_emptystring(self):
         ]
         self.assertEqual(serializer.data, expected)
 
+    def test_null_uuid_foreign_key_serializes_as_none(self):
+        source = NullableUUIDForeignKeySource(name='Source')
+        serializer = NullableUUIDForeignKeySourceSerializer(source)
+        data = serializer.data
+        self.assertEqual(data["target"], None)
+
+    def test_nullable_uuid_foreign_key_is_valid_when_none(self):
+        data = {"name": "Source", "target": None}
+        serializer = NullableUUIDForeignKeySourceSerializer(data=data)
+        self.assertTrue(serializer.is_valid(), serializer.errors)
+
 
 class PKNullableOneToOneTests(TestCase):
     def setUp(self):