[1.8.x] Fixed #24320 - Used field.value_to_string() in serialization of foreign key.

This fixes serialization of a ForeignKey to a UUIDField as the
test indicates.

Backport of 5c995dcfc2 from master

django/core/serializers/python.py

@@ -63,6 +63,8 @@ class Serializer(base.Serializer):
                 value = None
         else:
             value = getattr(obj, field.get_attname())
+            if not is_protected_type(value):
+                value = field.value_to_string(obj)
         self._current[field.name] = value
 
     def handle_m2m_field(self, obj, field):

tests/serializers_regress/models.py

@@ -284,6 +284,10 @@ class UUIDData(models.Model):
     data = models.UUIDField(primary_key=True)
 
 
+class FKToUUID(models.Model):
+    data = models.ForeignKey(UUIDData)
+
+
 class ComplexModel(models.Model):
     field1 = models.CharField(max_length=10)
     field2 = models.CharField(max_length=10)

tests/serializers_regress/tests.py

@@ -29,15 +29,16 @@ from .models import (
     BooleanData, BooleanPKData, CharData, CharPKData, ComplexModel, DateData,
     DateTimeData, DecimalData, DecimalPKData, EmailData, EmailPKData,
     ExplicitInheritBaseModel, FileData, FilePathData, FilePathPKData, FKData,
-    FKDataNaturalKey, FKDataToField, FKDataToO2O, FKSelfData, FloatData,
-    FloatPKData, GenericData, GenericIPAddressData, GenericIPAddressPKData,
-    InheritAbstractModel, InheritBaseModel, IntegerData, IntegerPKData,
-    Intermediate, IPAddressData, IPAddressPKData, LengthModel, M2MData,
-    M2MIntermediateData, M2MSelfData, ModifyingSaveData, NaturalKeyAnchor,
-    NullBooleanData, O2OData, PositiveIntegerData, PositiveIntegerPKData,
-    PositiveSmallIntegerData, PositiveSmallIntegerPKData, ProxyBaseModel,
-    ProxyProxyBaseModel, SlugData, SlugPKData, SmallData, SmallPKData, Tag,
-    TextData, TimeData, UniqueAnchor, UUIDData,
+    FKDataNaturalKey, FKDataToField, FKDataToO2O, FKSelfData, FKToUUID,
+    FloatData, FloatPKData, GenericData, GenericIPAddressData,
+    GenericIPAddressPKData, InheritAbstractModel, InheritBaseModel,
+    IntegerData, IntegerPKData, Intermediate, IPAddressData, IPAddressPKData,
+    LengthModel, M2MData, M2MIntermediateData, M2MSelfData, ModifyingSaveData,
+    NaturalKeyAnchor, NullBooleanData, O2OData, PositiveIntegerData,
+    PositiveIntegerPKData, PositiveSmallIntegerData,
+    PositiveSmallIntegerPKData, ProxyBaseModel, ProxyProxyBaseModel, SlugData,
+    SlugPKData, SmallData, SmallPKData, Tag, TextData, TimeData, UniqueAnchor,
+    UUIDData,
 )
 
 try:
@@ -203,6 +204,7 @@ im_obj = (im_create, im_compare)
 o2o_obj = (o2o_create, o2o_compare)
 pk_obj = (pk_create, pk_compare)
 inherited_obj = (inherited_create, inherited_compare)
+uuid_obj = uuid.uuid4()
 
 test_data = [
     # Format: (data type, PK value, Model Class, data)
@@ -361,7 +363,8 @@ The end."""),
     # The end."""),
     # (pk_obj, 770, TimePKData, datetime.time(10, 42, 37)),
     # (pk_obj, 790, XMLPKData, "<foo></foo>"),
-    (pk_obj, 791, UUIDData, uuid.uuid4()),
+    (pk_obj, 791, UUIDData, uuid_obj),
+    (fk_obj, 792, FKToUUID, uuid_obj),
 
     (data_obj, 800, AutoNowDateTimeData, datetime.datetime(2006, 6, 16, 10, 42, 37)),
     (data_obj, 810, ModifyingSaveData, 42),