Django REST Framework: nested serializer not properly validating data

I'm new to DRF (and Django) and trying to create a nested serializer which is able to validate the following request data:

{
    "code": "12345",
    "city": {
        "name": "atlanta",
        "state": "Georgia"
    },
    "subregion": {
        "name": "otp north"
    }
}

To simplify things for the client, I'd like this single request to create multiple records in the database:

• A City (if a matching one doesn't already exist)
• A Subregion (if a matching one doesn't already exist)
• A CodeLog which references a city and (optionally) a subregion

Models:

class City(models.Model):
    name = models.CharField(max_length=75, unique=True)
    state = models.CharField(max_length=50, blank=True)

class Subregion(models.Model):
    city = models.ForeignKey(City)
    name = models.CharField(max_length=75)

class CodeLog(models.Model):
    code = models.CharField(max_length=10)
    city = models.ForeignKey(City)
    subregion = models.ForeignKey(Subregion, blank=True, null=True)

Serializers:

class CitySerializer(serializers.ModelSerializer):
    class Meta:
        model = City
        fields = ('name', 'state')

class SubregionSerializer(serializers.ModelSerializer):
    class Meta:
        model = Subregion
        fields = ('name',)

class CodeLogSerializer(serializers.ModelSerializer):
    city = CitySerializer()
    subregion = SubregionSerializer(required=False)

    class Meta:
        model = CodeLog
        fields = ('code', 'city', 'subregion')

    # This is where I'm having troubles
    def create(self, validated_data):
        city_data = validated_data.pop('city', None)
        subregion_data = validated_data.pop('subregion', None)
        if city_data:
            city = City.objects.get_or_create(**city_data)[0]
        subregion = None
        if subregion_data:
            subregion = Subregion.objects.get_or_create(**subregion_data)[0]

        code_log = CodeLog(
            code=validated_data.get('code'),
            city=city,
            subregion=subregion
        )
        code_log.save()
        return code_log

View:

class CodeLogList(APIView):
    serializer_class = CodeLogSerializer

    def post(self, request):
        serializer = self.serializer_class(data=request.data)
        if serializer.is_valid():
            serializer.save()
            return Response(serializer.data, status=status.HTTP_201_CREATED)
        return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)

I was able to get it working with a flat request structure, nested serializers are proving difficult to grasp. Am I on the right track, or is there a more ideal structure that would work better? Any help is greatly appreciated!