Serverside sorting with ForeignKey produces duplicate about django-rest-framework-datatables

[alfonsrv]

Ah thinking SQL has increasingly become an up-hill battle once I started to delve into server-specific SQL-functions. Pair that with the Django query-specifics and I'm in awe by all the clever ways people come up with querying for things. I seriously don't know how people are doing it.

My favorite being queries like Model.objects.filter(last_datetime__lte=Now() + timedelta(seconds=1) * F("interval"))

---

Anyways, I solved this issue building on your answer by expanding the viewset's queryset as follows:

class ArtistViewSet():
   ...

    def get_queryset(self):
        queryset = super().get_queryset()
        albums = Album.objects.filter(artist=OuterRef('pk')).order_by('-created')
        queryset = queryset.annotate(
            latest_album=Subquery(albums.values('name')[:1])
        )
        return queryset

Thanks.

[matthewhegarty]

You can get the latest album (by year) for each artist using a subquery (added in overridden get_queryset()):

class AlbumViewSet(viewsets.ModelViewSet):
    queryset = Album.objects.none()
    serializer_class = AlbumSerializer

    def get_queryset(self):
        latest_album_qry = (Album.objects.filter(artist=OuterRef("artist_id")).order_by("-year"))[:1]
        qs = Album.objects.all().order_by("pk")
        qs = qs.filter(id__in=latest_album_qry.values("pk"))
        return qs.prefetch_related("artist", "genres")

    def get_options(self):
        return get_album_options()

    class Meta:
        datatables_extra_json = ('get_options', )

This generates the following sql:

SELECT "albums_album"."id", "albums_album"."name", "albums_album"."rank", "albums_album"."year", "albums_album"."artist_id" 
FROM "albums_album" INNER JOIN "albums_artist" ON ("albums_album"."artist_id" = "albums_artist"."id") 
WHERE "albums_album"."id" IN (
   SELECT U0."id" FROM "albums_album" U0 WHERE U0."artist_id" = "albums_album"."artist_id" ORDER BY U0."year" DESC LIMIT 1
) 
ORDER BY "albums_artist"."name" ASC LIMIT 10;

[alfonsrv]

Any way to get the Artist as the primary queryset aka use an ArtistViewSet? Say you'd want to display mainly the artist's information along with the latest Album; this works, but messed up the hierarchy.

Another option would obviously be to de-normalize the Album, saving it on the Artist directly whenever a newer album is added via a signal, but that is kind of less-than-ideal. To emulate that I added the property, as it allows to keep the Artist as the main model when querying and prevents logic duplication among multiple parts of the project / scattered querysets.

[matthewhegarty]

I'm not sure what you mean, but I would suggest you write the query you want as SQL SELECT first, then port that over to a Django query. You might want to return all Artists, and then outer join the latest album onto each (query on Artist table); or you might just want the latest Album with associated Artist information (query on Album table).