Question

I have a bunch of objects that have a value and a date field:

obj1 = Obj(date='2009-8-20', value=10)
obj2 = Obj(date='2009-8-21', value=15)
obj3 = Obj(date='2009-8-23', value=8)

I want this returned:

[10, 15, 0, 8]

or better yet, an aggregate of the total up to that point:

[10, 25, 25, 33]

I would be best to get this data directly from the database, but otherwise I can do the totaling pretty easily with a forloop.

I’m using Django’s ORM and also Postgres

edit:

Just to note, that my example only covers a few days, but in practice, I have hundreds of objects covering a couple decades… What I’m trying to do is create a line graph showing how the sum of all my objects has grown over time (a very long time)

Answer 1

This one isn’t tested, since it’s a bit too much of a pain to set up a Django table to test with:

from datetime import date, timedelta
# http://www.ianlewis.org/en/python-date-range-iterator
def datetimeRange(from_date, to_date=None):
    while to_date is None or from_date <= to_date:
        yield from_date
        from_date = from_date + timedelta(days = 1)

start = date(2009, 8, 20)
end = date(2009, 8, 23)
objects = Obj.objects.filter(date__gte=start)
objects = objects.filter(date__lte=end)

results = {}
for o in objects:
    results[o.date] = o.value

return [results.get(day, 0) for day in datetimeRange(start, end)]

This avoids running a separate query for every day.