registration_id unique constraint fails on PUT (Update)

[mr-const]

We use GCMDeviceSerializer as nested serializer

class DataSerializer(serializers.ModelSerializer):
    gcm_device = GCMDeviceSerializer(allow_null=True, required=False)

and it always fails validation when we try to update our Data object with message: {"gcm_device":{"registration_id":["This field must be unique."]}}

It seems that following code executes on both create and update calls:

            # Work around an issue with validating the uniqueness of
            # registration ids of up to 4k
            'registration_id': {
                'validators': [
                    UniqueValidator(queryset=GCMDevice.objects.all())
                ]
            }

[jamaalscarlett]

This appears to be a known issue with writable nested serializers encode/django-rest-framework#2403