[Answered ]-DRF DefaultRouter pk/slug

You can use the MultipleLookupField mixin strategy and define a custom get_object method on your viewset for this.

class PostViewSet(viewsets.ModelViewSet):
    lookup_fields = ['pk', 'slug']
    # ...
    def get_object(self):
        if all(arg in self.kwargs for arg in self.lookup_fields):
            # detected the custom URL pattern; return the specified object
            qs = self.get_queryset()
            qs_filter = {field: self.kwargs[field] for field in self.lookup_fields}
            obj = get_object_or_404(qs, **qs_filter)
            self.check_object_permissions(self.request, obj)
            return obj
        else: # missing lookup fields
            raise InvalidQueryError("Missing fields")

class InvalidQueryError(APIException):
    status_code = status.HTTP_400_BAD_REQUEST

In this case, I just override get_object directly on the viewset. But you can make this a mixin class instead in order to easily include it into other viewsets.

The default router, however, will not automatically add this URL pattern to your app, so you’ll have to do that manually.

urlpatterns = [
    ...,
    path('api/post/<int:pk>/<str:slug>', views.PostViewSet.as_view()),
]

I dove into the source code of DefaultRouter and came up with a solution that passed my unittests.

DRF Routers source code

create a file routers.py

from rest_framework.routers import DefaultRouter

class CustomRouter(DefaultRouter):

def get_lookup_regex(self, viewset, lookup_prefix=''):
    combined_lookup_field = getattr(viewset, 'combined_lookup_field', None)

    if combined_lookup_field:
        multi_base_regex_list = []
        
        for lookup_field in combined_lookup_field:
            base_regex = '(?P<{lookup_prefix}>{lookup_value})'
            lookup_value = getattr(viewset, 'lookup_value_regex', '[^/.]+')
            multi_base_regex_list.append(
                base_regex.format(
                    lookup_prefix=lookup_field,
                    lookup_value=lookup_value
                )
            )

        return'/'.join(multi_base_regex_list)

    return super().get_lookup_regex(viewset, lookup_prefix)

replace lookup_field or add the following in views.py

class PostViewSet(viewsets.ModelViewSet):
    ...
    combined_lookup_field = ['pk', 'slug']

replace DefaultRouter with CustomRouter in urls.py

from .routers import CustomRouter

router = CustomRouter()
router.register('posts', PostViewSet, basename='post')

app_name = 'api'
urlpatterns = [
    path('', include(router.urls)),
]

And that’s it! It still incorporates all other functionality of DefaultRouter and will only use the added logic on views that have combined_lookup_field defined. It also supports @action functionality in the views.