How to achieve nested URL for OneToOne Relationship?

Question

I'm a little bit stuck with the following situation. I want to build a REST API for a shopping cart app using the Django Rest Framework, however, due to legacy requirements I need to work with nested URLs.

In general, I have two resources AppUsers and Carts. Both of the resources are available at the default /appUsers/ and /carts/ endpoints. I then tried using nested routers to get the cart detail view for a specific user to be addressable as /appUsers/app_user_pk/cart/ instead of /carts/pk/ since every AppUser can only have one cart anyway.

Here's my setup:

models.py

class AppUser(models.Model):
    _id = models.AutoField(primary_key=True)

    class Meta:
        default_related_name = 'app_users'


class Cart(models.Model):
    app_user = models.OneToOneField(
        'AppUser',
        on_delete=models.CASCADE,
        related_query_name='cart',
    )

    class Meta:
        default_related_name = 'carts'

    def __str__(self):
        return "{user} cart".format(user=self.app_user._id)

serializers.py

class AppUserSerializer(serializers.HyperlinkedModelSerializer):
    cart = serializers.HyperlinkedIdentityField(view_name='cart-detail')

    class Meta:
        model = AppUser
        fields = '__all__'


class CartSerializer(serializers.HyperlinkedModelSerializer):
    class Meta:
        model = Cart
        fields = '__all__'

views.py

class AppUserViewSet(viewsets.ModelViewSet):
    """
    Model viewset for AppUser model
    """
    queryset = AppUser.objects.all()
    serializer_class = AppUserSerializer


class CartViewSet(viewsets.ModelViewSet):
    """
    List all carts, or create new / edit existing product.
    """
    queryset = Cart.objects.all()
    serializer_class = CartSerializer

    def get_queryset(self):
        if 'app_user_pk' in self.kwargs:
            return Cart.objects.filter(app_user=self.kwargs['app_user_pk'])
        return Cart.objects.all()

urls.py

router = DefaultRouter()

router.register(r'appUsers', views.AppUserViewSet)
router.register(r'carts', views.CartViewSet)
urlpatterns = [
    url(r'^', include(router.urls)),
    url(r'^appUsers/(?P<app_user_pk>[0-9]+)/cart/$', views.CartViewSet.as_view({'get': 'retrieve'}), name='cart-detail'),
]

However, I can't get it to work, whenever I try something new I run into a different issue. So I was wondering what the best way is to achieve such task?

Essentially I just want to have cart-detail view for the one shopping cart the an AppUser can have under /appUsers/app_user_pk/cart/

SOLUTION:

I used the accepted answer to deal with the issue above. Additionally I had another ModelViewSet located at /appUsers/app_user_pk/cart/products, which I then registered using a NestedDefaultRouter from drf-nested-routers at cart/products like this:

cart_products_router = routers.NestedDefaultRouter(router, r'appUsers', lookup='app_user')
cart_products_router.register(r'cart/products', views.CartProductViewSet, base_name='cartproduct')

Answer 1

You can create a custom method to AppUserViewSet:

class AppUserViewSet(viewsets.ModelViewSet):
    """
    Model viewset for AppUser model
    """
    queryset = AppUser.objects.all()
    serializer_class = AppUserSerializer

    @action(detail=True)
    def cart(self, request, pk):
        obj = Cart.objects.get(app_user_id=pk)
        serializer = CartSerializer(obj)
        return Response(serializer.data)