Почему данные сериализуются значениями null?

Question

[mezigar]

Есть модели:

class City(models.Model):
    name = models.CharField(max_length=50, default='')

    def __str__(self):
        return self.name

class Street(models.Model):
    name = models.CharField(max_length=50)
    city = models.ForeignKey(City, on_delete=models.CASCADE)

    def __str__(self):
        return f"{self.city}: {self.name}"

class Shop(models.Model):
    name = models.CharField(max_length=50)
    city = models.ForeignKey(City, on_delete=models.SET_NULL, null=True)
    street = models.ForeignKey(Street, on_delete=models.SET_NULL, null=True)
    house = models.SmallIntegerField()
    opening_time = models.TimeField()
    closing_time = models.TimeField()

    def __str__(self):
        return f"{self.name} по адрессу {self.street} - {self.house}. Работает с {self.opening_time} до {self.closing_time}"

Есть сериализаторы:

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


class StreetSerializer(serializers.ModelSerializer):
    city = serializers.StringRelatedField(many=False)

    class Meta:
        model =  Street
        fields = ['id', 'name', 'city']


class ShopSerializer(serializers.ModelSerializer):
    city = CitySerializer(read_only=True)
    street = StreetSerializer(read_only=True)
    class Meta:
        model = Shop
        fields = ['id', 'name', 'city', 'street', 'house', 'opening_time', 'closing_time']

Имеется такое вью:

class ShopView(APIView):
    def post(self, request):
        shop = request.data.get('shop')
        serializer = ShopSerializer(data=shop)
        if serializer.is_valid(raise_exception=True):
            shop_saved = serializer.save()
        return Response({"success": "Shop '{}' created successfully".format(shop_saved.name)})

    def get(self, request):
        shops = Shop.objects.all()
        serializer = ShopSerializer(shops, many=True)
        return Response({"shops": serializer.data})

При POST запросе в Postman следующего вида:

{"shop": {
    "id": 1,
    "name": "Globus",
    "city": {"name":"Ryazan"},
    "street": {"name": "Lenina", "city": {"name":"Ryazan"}},
    "house": 2,
    "opening_time": "8:00",
    "closing_time": "22:00"
}
}

Соответствующие поля street и city равно null

{"id":2,
             "name":"Globus2",
              "city":null,
              "street":null,
              "house":2,
              "opening_time":"08:00:00",
              "closing_time":"22:00:00"
}]}

Не понимаю, почему сериализатор не отрабатывает?