Serializer doesn't save data into DB, but response is 200

I need to take data by using keys from different models, and then put them together and save into another model.

views.py:

    def create_order(self, request):
        instance = self.get_object()
        serializer = CreateOrderSerializer(instance, data=request.data)
        serializer.is_valid(raise_exception=True)
        serializer.save()
        print(serializer.data)
        return Response(serializer.data)

serializers.py:

class CreateOrderSerializer(serializers.ModelSerializer):
    user_name = serializers.ReadOnlyField(source="author.username", required=False, allow_null=True)
    book = serializers.ReadOnlyField(source="book_name", required=False, allow_null=True)

    class Meta:
        model = order
        fields = ("id", "user_name", "book")

I can access and operate with data which i need, and i get 200 response when make POST request, but data doesn't save. By the way, when i create "order" in admin menu everythink works fine, thus i assume something is going wrong in either my view or serializer.

Finally i fixed this.

serializer.py

class CreateOrderSerializer(serializers.ModelSerializer):
    user_name = serializers.CharField(source="author.username", required=False, allow_null=True)
    book = serializers.CharField(source="book_name", required=False, allow_null=True)

    class Meta:
        model = order
        fields = ("id", "user_name", "book")

views.py

    def create_order(self, request):
        instance = self.get_object()
        serializer = CreateOrderSerializer(instance, data=request.data)
        serializer.is_valid(raise_exception=True)
        user_id = request.user
        order.objects.create(
            user=user_id,
            book=instance,
        )
        return Response(serializer.data)