I'm trying to add additional data to an aggregated query, like so:
(there are many MyModels for any date, and I want to sum up
MyModel.value for every single date, and then do a calculation on that
sum)
qs = MyModel.objects.values('date').annotate(count=Sum('value'))
for item in qs:
item['kilocount'] = item['count'] / 1000
Now, it sounds reasonable to do that directly in the database:
MyModel.objects.values('date').annotate(count=Sum('value')).extra(select={'kilocount':
'"count" / 1000'))
According to the docs and my tests, .values() discards all .extra()
queries:
"If you use a values() clause after an extra() call, any fields
defined by a select argument in the extra() must be explicitly
included in the values() call. Any extra() call made after a values()
call will have its extra selected fields ignored."
If I try to fix this as proposed in the last paragraph, I end up with
the following:
MyModel.objects.extra(select={'kilocount': '"count" /
1000')).values('date', 'kilocount').annotate(count=Sum('value'))
which in turn raises a DatabaseError("misuse of aggregate: SUM()")
from an erroneous SQL query (in the GROUP BY part):
SELECT ("count" / 1000) AS "kilocount", "mymodel"."date",
SUM("mymodel"."value") AS "count" FROM "mymodel" GROUP BY
"mymodel"."date", "mymodel"."date", ("count" / 1000)
Is there no way to do this through the ORM on the database?
(Another solution would probably be a custom template filter, but that
doesn't strike me as too idiomatic.)
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