Annotate a QuerySet with the latest value from a filtered join

Question

How can I, in a single expression, get the latest value from the filtered results of a many-to-one relationship to annotate a Django Quer

Answer 1

You should be able to do this with the new Subquery functionality, which is also available as a backport to 1.8+.

try:
    from django.db.models.expressions import Subquery, OuterRef
except ImportError:
    from django_subquery.expressions import Subquery, OuterRef

class LoremAdmin(admin.ModelAdmin):
    # …

    def get_queryset(request):
        queryset = super().get_queryset(request)
        status_event_per_lorem = LoremStatusEvent.objects.filter(
            lorem=OuterRef('pk'))
        latest_status_event_per_lorem = (
            status_event_per_lorem.order_by(
                'pk', '-created').distinct('pk'))
        latest_status_event_for_ipsum_foo = (
            latest_status_event_per_lorem.filter(ipsum='foo'))
        latest_status_event_for_ipsum_bar = (
            latest_status_event_per_lorem.filter(ipsum='bar'))
        queryset = queryset.annotate(
            foo_status=Subquery(
                latest_status_event_for_ipsum_foo.values('status')[:1]), 
            bar_status=Subquery(
                latest_status_event_for_ipsum_bar.values('status')[:1]), 
        )

        return queryset