How to serialize the foreign key field in django rest framework

I work on a project with DRF where I'm getting serializer data as follows which is absolutely fine:

{
    "message": "Updated Successfully",
    "status": 200,
    "errors": {},
    "data": {
        "id": 8,
        "user": 2,
        "item": 1,
        "quantity": 4,
        "created_at": "2021-08-11T13:49:27.391939Z",
        "updated_at": "2021-08-11T13:51:07.229794Z"
    }
}

but I want to get as follows:

{
    "message": "Updated Successfully",
    "status": 200,
    "errors": {},
    "data": {
        "id": 8,
        "user": "user name",
        "item": "product name",
        "price: "3.44",
        "quantity": 4,
        "created_at": "2021-08-11T13:49:27.391939Z",
        "updated_at": "2021-08-11T13:51:07.229794Z"
    }
}

I tried using DRF RelatedField and PrimaryKryRelatedField but in all these cases I need to make corresponding fields as read_only=True which I want to skip.

I also tried with depth = 1 which gives entire details

My Model:

class Cart(models.Model):
    user = models.ForeignKey(User, on_delete=models.SET_NULL, null=True, blank=True)
    item = models.ForeignKey(Product, on_delete=models.SET_NULL, null=True, blank=True)
    quantity = models.IntegerField(null=False)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    def __str__(self):
        return "{} - {} - {} - {} - {}".format(self.user,
                                               self.item,
                                               self.quantity,
                                               self.created_at,
                                               self.updated_at)

My serializer:

class CartSerializer(serializers.ModelSerializer):
    user = serializers.PrimaryKeyRelatedField(source='user.first_name' ,queryset=User.objects.all(), many=False)
    class Meta:
        model = Cart
        fields = ['id', 'user', 'item', 'quantity', 'created_at', 'updated_at']

My View:

class CartViewSet(viewsets.ModelViewSet):
    queryset = Cart.objects.all().order_by('id')
    serializer_class = CartSerializer

    def list(self, request, *args, **kwargs):
        queryset = self.get_queryset()
        serializer = self.serializer_class(
            queryset, context={'request': request}, many=True)
        data = serializer.data
        context = {
            'message': 'Listed Successfully',
            'count': queryset.count(),
            'errors': False,
            'data': data,
        }
        return Response(context, status=status.HTTP_200_OK)

    def create(self, request, *args, **kwargs):
        if not request.data:
            return Response("No data", status=status.HTTP_400_BAD_REQUEST)
        serializer = self.get_serializer(data=request.data)
        if serializer.is_valid():
            self.perform_create(serializer)
        context = {
            'message': 'Created Successfully',
            'status': status.HTTP_201_CREATED,
            'errors': serializer.errors,
            'data': serializer.data,
        }
        return Response(context)

    def update(self, request, *args, **kwargs):
        partial = kwargs.pop('partial', False)
        instance = self.get_object()

        serializer = self.get_serializer(
            instance, data=request.data, partial=partial)
        if serializer.is_valid():
                self.perform_update(serializer)
        context = {
            'message': 'Updated Succesfully',
            'status': status.HTTP_200_OK,
            'errors': serializer.errors,
            'data': serializer.data,
        }
        return Response(context)

    def destroy(self, request, *args, **kwargs):
        instance = self.get_object()
        self.perform_destroy(instance)
        context = {
            'message': 'Deleted Succesfully',
            'status': status.HTTP_204_NO_CONTENT,
            'errors': False,
        }
        return Response(context)
        

    @action(methods=['get'], detail=False, url_path='checkout/(?P<userId>[^/.]+)', url_name='checkout')
    def checkout(self, request, *args, **kwargs):

        try:
            user = User.objects.get(pk=int(kwargs.get('userId')))
        except Exception as e:
            return Response(status=status.HTTP_404_NOT_FOUND, data={'Error': str(e)})

        cart_helper = CartHelper(user)
        checkout_details = cart_helper.prepare_cart_for_checkout()

        if not checkout_details:
            return Response(status=status.HTTP_404_NOT_FOUND,
                            data={'error': 'Cart of user is empty.'})

        return Response(status=status.HTTP_200_OK, data={'checkout_details': checkout_details})

Please if anyone can help, it would be greatly appreciated. Thanks

You can make use of to_representation()

Give this a try

class CartSerializer(serializers.ModelSerializer):

    class Meta:
        model = Cart

    def to_representation(self, instance):
        representation = dict()
        representation["id"] = instance.id
        representation["user"] = instance.user.username # see this
        representation["item"] = instance.item.name
        representation["quantity"] = instance.quantity
        representation["created_at"] = instance.created_at
        representation["updated_at"] = instance.updated_at

        return representation

But instead if you want to get the full user details in user filed of json response then try this

representation["user"] = UserSerializer(instance.user).data 

NB: You may have to change instance.field_name(s) accordingly