Improve handling of 'empty' values for ChoiceField

The empty value defaults back to '' (for backwards-compatibility) but
is changed automatically to None for ModelSerializers if the `null`
property is set on the db field.

rest_framework/fields.py

@@ -497,6 +497,7 @@ class ChoiceField(WritableField):
     }
 
     def __init__(self, choices=(), *args, **kwargs):
+        self.empty = kwargs.pop('empty', '')
         super(ChoiceField, self).__init__(*args, **kwargs)
         self.choices = choices
         if not self.required:
@@ -537,9 +538,10 @@ class ChoiceField(WritableField):
         return False
 
     def from_native(self, value):
-        if value in validators.EMPTY_VALUES:
-            return None
-        return super(ChoiceField, self).from_native(value)
+        value = super(ChoiceField, self).from_native(value)
+        if value == self.empty or value in validators.EMPTY_VALUES:
+            return self.empty
+        return value
 
 
 class EmailField(CharField):

rest_framework/serializers.py

@@ -794,6 +794,8 @@ class ModelSerializer(Serializer):
         # TODO: TypedChoiceField?
         if model_field.flatchoices:  # This ModelField contains choices
             kwargs['choices'] = model_field.flatchoices
+            if model_field.null:
+                kwargs['empty'] = None
             return ChoiceField(**kwargs)
 
         # put this below the ChoiceField because min_value isn't a valid initializer

rest_framework/tests/test_fields.py

@@ -42,6 +42,31 @@ class TimeFieldModelSerializer(serializers.ModelSerializer):
         model = TimeFieldModel
 
 
+SAMPLE_CHOICES = [
+    ('red', 'Red'),
+    ('green', 'Green'),
+    ('blue', 'Blue'),
+]
+
+
+class ChoiceFieldModel(models.Model):
+    choice = models.CharField(choices=SAMPLE_CHOICES, blank=True, max_length=255)
+
+
+class ChoiceFieldModelSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = ChoiceFieldModel
+
+
+class ChoiceFieldModelWithNull(models.Model):
+    choice = models.CharField(choices=SAMPLE_CHOICES, blank=True, null=True, max_length=255)
+
+
+class ChoiceFieldModelWithNullSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = ChoiceFieldModelWithNull
+
+
 class BasicFieldTests(TestCase):
     def test_auto_now_fields_read_only(self):
         """
@@ -667,34 +692,53 @@ class ChoiceFieldTests(TestCase):
     """
     Tests for the ChoiceField options generator
     """
-
-    SAMPLE_CHOICES = [
-        ('red', 'Red'),
-        ('green', 'Green'),
-        ('blue', 'Blue'),
-    ]
-
     def test_choices_required(self):
         """
         Make sure proper choices are rendered if field is required
         """
-        f = serializers.ChoiceField(required=True, choices=self.SAMPLE_CHOICES)
-        self.assertEqual(f.choices, self.SAMPLE_CHOICES)
+        f = serializers.ChoiceField(required=True, choices=SAMPLE_CHOICES)
+        self.assertEqual(f.choices, SAMPLE_CHOICES)
 
     def test_choices_not_required(self):
         """
         Make sure proper choices (plus blank) are rendered if the field isn't required
         """
-        f = serializers.ChoiceField(required=False, choices=self.SAMPLE_CHOICES)
-        self.assertEqual(f.choices, models.fields.BLANK_CHOICE_DASH + self.SAMPLE_CHOICES)
+        f = serializers.ChoiceField(required=False, choices=SAMPLE_CHOICES)
+        self.assertEqual(f.choices, models.fields.BLANK_CHOICE_DASH + SAMPLE_CHOICES)
+
+    def test_invalid_choice_model(self):
+        s = ChoiceFieldModelSerializer(data={'choice' : 'wrong_value'})
+        self.assertFalse(s.is_valid())
+        self.assertEqual(s.errors,  {'choice': [u'Select a valid choice. wrong_value is not one of the available choices.']})
+        self.assertEqual(s.data['choice'], '')
+
+    def test_empty_choice_model(self):
+        """
+        Test that the 'empty' value is correctly passed and used depending on the 'null' property on the model field.
+        """
+        s = ChoiceFieldModelSerializer(data={'choice' : ''})
+        self.assertTrue(s.is_valid())
+        self.assertEqual(s.data['choice'], '')
+
+        s = ChoiceFieldModelWithNullSerializer(data={'choice' : ''})
+        self.assertTrue(s.is_valid())
+        self.assertEqual(s.data['choice'], None)
 
     def test_from_native_empty(self):
         """
-        Make sure from_native() returns None on empty param.
+        Make sure from_native() returns an empty string on empty param by default.
         """
-        f = serializers.ChoiceField(choices=self.SAMPLE_CHOICES)
-        result = f.from_native('')
-        self.assertEqual(result, None)
+        f = serializers.ChoiceField(choices=SAMPLE_CHOICES)
+        self.assertEqual(f.from_native(''), '')
+        self.assertEqual(f.from_native(None), '')
+
+    def test_from_native_empty_override(self):
+        """
+        Make sure you can override from_native() behavior regarding empty values.
+        """
+        f = serializers.ChoiceField(choices=SAMPLE_CHOICES, empty=None)
+        self.assertEqual(f.from_native(''), None)
+        self.assertEqual(f.from_native(None), None)
 
 
 class EmailFieldTests(TestCase):