Fix returning None when allow_none is True in CharField

rest_framework/fields.py

@@ -475,8 +475,12 @@ class CharField(WritableField):
         if isinstance(value, six.string_types):
             return value
 
-        if value is None and not self.allow_none:
+        if value is None:
+            if not self.allow_none:
                 return ''
+            else:
+                # return None implicity because smart_text(None) == 'None'
+                return None
 
         return smart_text(value)
 

tests/test_fields.py

@@ -54,6 +54,10 @@ class ChoiceFieldModel(models.Model):
     choice = models.CharField(choices=SAMPLE_CHOICES, blank=True, max_length=255)
 
 
+class NullableCharFieldModel(models.Model):
+    char = models.CharField(null=True, blank=True, max_length=4)
+
+
 class ChoiceFieldModelSerializer(serializers.ModelSerializer):
     class Meta:
         model = ChoiceFieldModel
@@ -1004,6 +1008,20 @@ class BooleanField(TestCase):
         self.assertFalse(BooleanRequiredSerializer(data={}).is_valid())
 
 
+class ModelCharField(TestCase):
+    """
+        Tests for CharField
+    """
+    def test_none_serializing(self):
+        class CharFieldSerializer(serializers.ModelSerializer):
+            class Meta:
+                model = NullableCharFieldModel
+        serializer = CharFieldSerializer(data={'char': None})
+        self.assertTrue(serializer.fields['char'].allow_none)
+        self.assertTrue(serializer.is_valid())
+        self.assertIsNone(serializer.data['char'])
+
+
 class SerializerMethodFieldTest(TestCase):
     """
         Tests for the SerializerMethodField field_to_native() behavior