#21524: Annotation and .extra with group function breaks at database level
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Reporter: eu@… | Owner: nobody
Type: Uncategorized | Status: new
Component: Database layer | Version: 1.4
(models, ORM) | Resolution:
Severity: Normal | Triage Stage:
Keywords: oracle | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by shai):
The solution suggested is indeed dirty -- and bogus (I can think up valid
queries it breaks, the most trivial of which adds constant 'sum()' in the
extra columns).
In a similar case, I did something like
{{{#!python
def annotate_percents(qset):
class AnnotatedQuerySet(qset.__class__):
def iterator(self):
for obj in super(AnnotatedQuerySet, self).iterator():
obj.closed_percent = obj.closed/obj.total * 100
obj.opened_percent = obj.opened/obj.total * 100
yield obj
qset.__class__ = AnnotatedQuerySet
return qset
}}}
Now, in the case you have with AVG you may really need to do things in the
database; you can still do this efficiently by judicious use of
`prefetch_related`.
It can allow you to do things that look like you add a new related query
per object returned, but really only cost you one more database access per
the whole queryset.
I hope this helps; note that even if something like your use-case ends up
supported, it will probably not be in Django 1.4.x.
--
Ticket URL: <https://code.djangoproject.com/ticket/21524#comment:3>
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