django – prefetch only the newest record?

LinkData.objects.all().order_by('-id')[0] is not a queryset, it is an model object, hence your error.

You could try LinkData.objects.all().order_by('-id')[0:1] which is indeed a QuerySet, but it’s not going to work. Given how prefetch_related works, the queryset argument must return a queryset that contains all the LinkData records you need (this is then further filtered, and the items in it joined up with the LinkTarget objects). This queryset only contains one item, so that’s no good. (And Django will complain “Cannot filter a query once a slice has been taken” and raise an exception, as it should).

Let’s back up. Essentially you are asking an aggregation/annotation question – for each LinkTarget, you want to know the most recent LinkData object, or the ‘max’ of an ‘id’ column. The easiest way is to just annotate with the id, and then do a separate query to get all the objects.

So, it would look like this (I’ve checked with a similar model in my project, so it should work, but the code below may have some typos):

linktargets = (LinkTargets.objects
               .filter(dashboard=True)
               .annotate(most_recent_linkdata_id=Max('linkdata_set__id'))

# Now, if we need them, lets collect and get the actual objects
linkdata_ids = [t.most_recent_linkdata_id for t in linktargets]
linkdata_objects = LinkData.objects.filter(id__in=linkdata_ids)

# And we can decorate the LinkTarget objects as well if we want:

linkdata_d = {l.id: l for l in linkdata_objects}
for t in linktargets:
    if t.most_recent_linkdata_id is not None:
        t.most_recent_linkdata = linkdata_d[t.most_recent_linkdata_id]

I have deliberately not made this into a prefetch that masks linkdata_set, because the result is that you have objects that lie to you – the linkdata_set attribute is now missing results. Do you really want to be bitten by that somewhere down the line? Best to make a new attribute that has just the thing you want.

The following works on PostgreSQL. I understand it won’t help OP, but it might be useful to somebody else.

from django.db.models import Count, Prefetch
from .models import LinkTargets, LinkData

link_data_qs = LinkData.objects.order_by(
    'link_target__id',
    '-id',
).distinct(
    'link_target__id',
)

qs = LinkTargets.objects.prefetch_related(
    Prefetch(
        'linkdata_set',
        queryset=link_data_qs,
    )
).all()

Tricky, but it seems to work:

class ForeignKeyAsOneToOneField(models.OneToOneField):
    def __init__(self, to, on_delete, to_field=None, **kwargs):
        super().__init__(to, on_delete, to_field=to_field, **kwargs)
        self._unique = False

class LinkData(models.Model):
    # link_target = models.ForeignKey(LinkTargets, verbose_name="Link Target", on_delete=models.PROTECT)
    link_target = ForeignKeyAsOneToOneField(LinkTargets, verbose_name="Link Target", on_delete=models.PROTECT, related_name='linkdata_helper')
    interface_description = models.CharField(max_length=200, verbose_name='Interface Description', blank=True, null=True)


link_data = LinkTargets.objects.filter(dashboard=True) 
                               .prefetch_related(
                                    Prefetch(
                                        'linkdata_helper',
                                        queryset=LinkData.objects.all().order_by('-id'),
                                        'linkdata'
                                    )
                                )

# Now you can access linkdata:
link_data[0].linkdata

Ofcourse with this approach you can’t use linkdata_helper to get related objects.

This is not a direct answer to you question, but solves the same problem. It is possible annotate newest object with a subquery, which I think is more clear. You also don’t have to do stuff like Max("id") to limit the prefetch query.

It makes use of django.db.models.functions.JSONObject (added in Django 3.2) to combine multiple fields:

MainModel.objects.annotate(
    last_object=RelatedModel.objects.filter(mainmodel=OuterRef("pk"))
    .order_by("-date_created")
    .values(
        data=JSONObject(
            id="id", body="body", date_created="date_created"
        )
    )[:1]
)