Django: Distinct foreign keys

Question

class Log:
 project = ForeignKey(Project)
 msg = CharField(...)
 date = DateField(...)

I want to select the four most recent Log entries where each

Answer 1

You need two querysets. The good thing is it still results in a single trip to the database (though there is a subquery involved).

latest_ids_per_project = Log.objects.values_list(
    'project').annotate(latest=Max('date')).order_by(
    '-latest').values_list('project')

log_objects = Log.objects.filter(
     id__in=latest_ids_per_project[:4]).order_by('-date')

This looks a bit convoluted, but it actually results in a surprisingly compact query:

SELECT "log"."id",
       "log"."project_id",
       "log"."msg"
       "log"."date"
FROM "log"
WHERE "log"."id" IN
    (SELECT U0."id"
     FROM "log" U0
     GROUP BY U0."project_id"
     ORDER BY MAX(U0."date") DESC
     LIMIT 4)
ORDER BY "log"."date" DESC