Django 1.0/1.1 rewrite of self join
Question
Is there a way to rewrite this query using the Django QuerySet object:
SELECT b.created_on, SUM(a.vote)
FROM votes a JOIN votes b ON a.created_on <= b.created_on
WHERE a.object_id = 1
GROUP BY 1
Where votes is a table, object_id is an int that occurs multiple times (foreign key - although that doesn't matter here), and created_on which is a datetime.
FWIW, this query allows one to get a score at any time in the past by summing up all previous votes on that object_id.
Solution
I'm pretty sure that query cannot be created with the Django ORM. The new Django aggregation code is pretty flexible, but I don't think it can do exactly what you want.
Are you sure that query works? You seem to be missing a check that b.object_id is 1.
This code should work, but it's more than one line and not that efficient.
from django.db.models import Sum
v_list = votes.objects.filter(object__id=1)
for v in v_list:
v.previous_score = votes.objects.filter(object__id=1, created_on__lte=v.created_on).aggregate(Sum('vote'))["vote__sum"]
Aggregation is only available in trunk, so you might need to update your django install before you can do this.
OTHER TIPS
Aggregation isn't the issue; the problem here is that Django's ORM simply doesn't do joins on anything that isn't a ForeignKey, AFAIK.
This is what I'm using now. Ironically, the sql is broken but this is the gist of it:
def get_score_over_time(self, obj):
"""
Get a dictionary containing the score and number of votes
at all times historically
"""
import pdb; pdb.set_trace();
ctype = ContentType.objects.get_for_model(obj)
try:
query = """SELECT b.created_on, SUM(a.vote)
FROM %s a JOIN %s b
ON a.created_on <= b.created_on
WHERE a.object_id = %s
AND a.content_type_id = %s
GROUP BY 1""" % (
connection.ops.quote_name(self.model._meta.db_table),
connection.ops.quote_name(self.model._meta.db_table),
1,
ctype.id,
)
cursor = connection.cursor()
cursor.execute(query)
result_list = []
for row in cursor.fetchall():
result_list.append(row)
except models.ObjectDoesNotExist:
result_list = None
return result_list