Django query with order_by, distinct and limit on Postgresql - django

Question

I have the following :
class Product(models.Model):
name = models.CharField(max_length=255)
class Action(models.Model):
product = models.ForeignKey(Product)
created_at = models.DateTimeField(auto_now_add=True)
I would like to retrieve the 10 most recent actions ordered by created_at DESC with distinct products.
The following is close to the result but still misses the ordering:
Action.objects.all().order_by('product_id').distinct('product_id')[:10]

Answer 1

Your solution seems like it's trying to do too much. It will also result in 2 separate SQL queries. This would work fine and with only a single query:
action_ids = Action.objects.order_by('product_id', '-created_at')\
.distinct('product_id').values_list('id', flat=True)
result = Action.objects.filter(id__in=action_ids)\
.order_by('-created_at')[:10]

Answer 2

EDIT: this solution works but Ross Lote's is cleaner
This is the way I finally did it, using Django Aggregation:
from django.db.models import Max
actions_id = Action.objects.all().values('product_id') \
.annotate(action_id=Max('id')) \
.order_by('-action_id')[:10] \
.values_list('action_id', flat=True)
result = Action.objects.filter(id__in=actions_id).order_by('-created_at')
By setting values('product_id') we do a group by on product_id.
With annotate() we can use order_by only on fields used in values() or annotate(). Since for each action the created_at field is automatically set to now, ordering on created_at is the same as ordering on id, using annotate(action_id=Max('id')).order_by('-action_id') is the right way.
Finnaly, we just need to slice our query [:10]
Hope this helps.