In Wagtail, consider the following page types:

class WorkshopPage(Page):
    category = models.ForeignKey(
    WorkshopCategory, on_delete=models.SET_NULL, blank=True, null=True
)

class WorkshopIndexPage(RoutablePageMixin, Page):
    subpage_types = ["WorkshopPage"]

In the wagtail admin, I need to order the subpages of WorkshopIndexPage by their category__name.

Since in the docs it says that setting ordering in a WorkshopPage Meta class will not work (it does not), I have tried the solution proposed here. However, when I do this inside my WorkshopIndexPage:

    def get_children(self):
        qs = WorkshopPage.objects.order_by("category__name")
        return qs

I get an error: Cannot combine queries on two different base models.

This seems like a simple use case but I can’t seem to get it to work.