Query latest votes per user per post in Django - django

Question

I have a voting system where a user can vote up or down on a post. The votes will be used in a calculation, so I need to store them in a log format, ie, I am saving each vote in it's own table.
Something like this:
class PointLog(models.Model):
post = models.ForeignKey(Post, db_index=True)
points = models.IntegerField(db_index=True)
user = models.ForeignKey(User, blank=True, null=True)
time = models.DateTimeField(auto_now_add=True, db_index=True)
data = models.IntegerField() # -1, 0 or 1
Now I need to display 20 Posts, together with the last vote the user did.
I am using django-rest-framework, so I can use a serializer field that looks like this; uservote = serializers.SerializerMethodField(), together with a function like:
def get_uservote(self, obj):
user = self.context['request'].user
vote = PointLog.objects.only('data').filter(user=user, post=obj).last()
return vote.data if vote else 0
But that will do a db-query 1 time per post, which I hope there is a better solution for.
I could save a db-query for each time the get_uservote is ran by saving the queryset in self.context, so that part is covered.
But how can I do a query that based on a list of items returns all the latest data from another table.
A start would be PointLog.objects.filter(user=user, post__in=posts), but what next? Is this even possible using raw SQL in 1 query?
Update 1:
PointLog.objects.filter(...).order_by('post__id').distinct('post__id') would kinda do it, except that I don't think I am guaranteed to get the newest vote. If I use order_by('pk') (or 'time'), I can't use distinct('post__id') as I will get an sql error (ProgrammingError: SELECT DISTINCT ON expressions must match initial ORDER BY expressions)

Answer 1

I found a solution that only added 1 extra query.
The full get_uservote that works would be;
def get_uservote(self, obj):
user = self.context['request'].user
if user.is_authenticated():
if not self.context.get('get_uservote_data'):
self.context['get_uservote_data'] = {i.post_id: i.data for i in PointLog.objects.filter(user=user, post__in=self.instance).order_by('post__id', '-pk').distinct('post__id')}
return self.context['get_uservote_data'].get(obj.id, 0)
else:
return 0 # Return 0 as "not voted" if not logged in
Note that we are doing a couple of tricks here;
Storing the data in self.context['get_uservote_data'] so we only haveto run the query one time.
Using i.post_id as the key for our data, and not i.post.id. Asking for i.post.id here would trigger 1 query per item.
self.instance is the queryset with all the posts we want to filter on.
This all resulted in a query that looks like this:
SELECT DISTINCT ON ("post_pointlog"."post_id") "post_pointlog"."id",
"post_pointlog"."post_id",
"post_pointlog"."data"
FROM "post_pointlog"
WHERE ( "post_pointlog"."user_id" = 20
AND "post_pointlog"."post_id" IN ( 1, 2, 3, 4 ) )
ORDER BY "post_pointlog"."post_id" ASC,
"post_pointlog"."id" DESC