根据相关字段最早实例的值过滤Django查询集

Question

Given the following models

class Thing(models.Model):
    ...

class Status(models.Model):
   thing = models.ForeignKey(Thing, ...)
   value = models.CharField(choices=(('foo', 'foo'), ('bar', 'bar')), ...)
   timestamp = models.DateTimeField(auto_now_add=True)

   class Meta:
      ordering = ('timestamp',)

Is it possible by using the Django ORM tools (without much raw SQL writing and post-query in-memory processing) to get the queryset of Thing s for which the latest timestamp has the value bar ?

In other words, I'd like to re-write the following code and achieve the same value for queryset :

thing_ids = []
for thing in Thing.objects.all():
    status = thing.status_set.last()
    if status is not None and status.value == 'bar':
        thing_ids.append(thing.id)

queryset = Thing.objects.filter(id__in=thing_ids)

Answer 1

Looking at Subquery documentation, ORM cookbook , and your if condition I think you might try something like following:

latest_status = Status.objects.filter(thing_id=OuterRef("pk")).order_by("-timestamp")
queryset = Thing.objects.annotate(
    lastest_status=Subquery(latest_status.values("value")[:1])
).filter(latest_status="bar")

Although I am not able to verify this right now. Anyway, very interesting question, I wasn't aware of those until now.