Choice Field validate the value before calling validate in serializer

[peeomid]

Checklist

• [x] I have verified that that issue exists against the master branch of Django REST framework.
• [x] I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• [x] This is not a usage question. (Those should be directed to the discussion group instead.)
• [x] This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• [x] I have reduced the issue to the simplest possible case.
• [ ] I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

I have a model with choice field (this is from fcm-django)

class FCMDevice(Device):
    DEVICE_TYPES = (
        (u'ios', u'ios'),
        (u'android', u'android'),
        (u'web', u'web')
    )

    type = models.CharField(choices=DEVICE_TYPES, max_length=10)

And in its serializer, I try to convert type to lower case

class FCMDeviceSerializer(ModelSerializer):
    class Meta(DeviceSerializerMixin.Meta):
        model = FCMDevice

    def validate_type(self, value):
        return value.lower()

Expected behavior

So when I pass value "IOS", it should convert to "ios" before validating with choices.

Actual behavior

It actually raises validation error before being validated

{'type': ['"IOS" is not a valid choice.']}

[carltongibson]

This is expected behaviour.

https://github.com/encode/django-rest-framework/blob/0da461710ad552c401a47b29b93e829ce879a696/rest_framework/serializers.py#L461-L468

Base field-level validation is applied before the additional serialiser-level methods are called.

Your best bet is to subclass the field and override to_internal_value to make the lower() call.

[peeomid]

Thanks a lot for such quick response @carltongibson, and thanks for pointing out the suggestion.

Just curious about this design though, as I thought serializer would be wrapper on top, before executing model level thing?