Django method to limit grouped queries - sql

Question

I want to return 3 results for every date and have the results ordered by the date and by a separate 'rating' column for each query
For example, my query would return something like this:
Event on Dec 1st rated 36
Event on Dec 1st rated 29
Event on Dec 1st rated 12
Event on Dec 2nd rated 45
Event on Dec 2nd rated 12
Event on Dec 2nd rated 9
Event on Dec 3rd rated 118
Event on Dec 3rd rated 15
Event on Dec 3rd rated 13
I know this should be possible using raw sql with something like this: SQL group - limit
But I am wondering whether there is a way to do this within the Django ORM in a single query or at least a way to make it as painless as possible if I do need to convert to a raw SQL query.
Edit:
Models are simple. Relevant fields are:
class Event(models.Model):
title = models.CharField(max_length=120)
day = models.DateField()
score = models.SmallIntegerField()

Answer 1

I tried to assemble a union of querysets, but django complained with:
AssertionError
Cannot combine queries once a slice has been taken.
This was the view code:
def home2(request):
dates_qs = Event.objects.values('day').order_by('day').distinct()
ev_qss = []
for date in dates_qs:
my_qs = Event.objects.filter(day=date['day']).order_by('score')[:3]
ev_qss.append(my_qs)
answer_qs = ev_qss[0]
for qs in ev_qss[1:]:
answer_qs |= qs
return render_to_response ('home2.html',
{'dates_qs':dates_qs,
'answer_qs':answer_qs},
RequestContext(request))
The error was issued for the line answer_qs |= qs, ie, wanting to take the union of answer_qs and qs. qs being the queryset of the scores for a date, limited to 3 results.
So I guess you're stuck with raw SQL. The raw SQL example you pointed to has data in several tables, and you have all your data in one table, so your SQL is a bit simpler:
SELECT sE.* FROM so_event AS sE
WHERE 3>(
SELECT COUNT(*)
FROM so_event iE
WHERE iE.day = sE.day AND
sE.score - iE.score < 0
)
ORDER BY sE.day ASC, sE.score DESC;
As now I know this is the query we are aiming for, I searched for django orm subqueries, and came across this SO article and answer:
How to django ORM with a subquery?
Which says a bunch of stuff, and hints that you might be able to do what you want with a different ORM (SQLAlchemy). I've heard good things about SQLAlchemy.