Django orm get latest for each group

Question

I am using Django 1.6 with Mysql.

I have these models:

class Student(models.Model):
     username = models.CharField(max_length=200, unique = True)

         


        

Answer 1

Here's an example using Greatest with a secondary annotate. I was facing and issue where annotate was returning duplicate records ( Examples ), but the last_message_time Greatest annotation was causing duplicates.

qs = (
            Example.objects.filter(
                Q(xyz=xyz)
            )
            .exclude(
                 Q(zzz=zzz)
            )
            # this annotation causes duplicate Examples in the qs
            # and distinct doesn't work, as expected
            # .distinct('id') 
            .annotate(
                last_message_time=Greatest(
                    "comments__created",
                    "files__owner_files__created",
                )
            )
            # so this second annotation selects the Max value of the various Greatest
            .annotate(
                last_message_time=Max(
                    "last_message_time"
                )
            )
            .order_by("-last_message_time")
    )

reference:

• https://docs.djangoproject.com/en/3.1/ref/models/database-functions/#greatest
• from django.db.models import Max

Answer 2

This should work on Django 1.2+ and MySQL:

Score.objects.annotate(
  max_date=Max('student__score__date')
).filter(
  date=F('max_date')
)

Answer 3

If your DB is postgres which supports distinct() on field you can try

Score.objects.order_by('student__username', '-date').distinct('student__username')

Answer 4

I believe this would give you the student and the data

Score.objects.values('student').annotate(latest_date=Max('date'))

If you want the full Score records, it seems you will have to use a raw SQL query: Filtering Django Query by the Record with the Maximum Column Value