#18375: F() doesn't work as expected across multijoin relations
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Reporter: FunkyBob | Owner: akaariai
Type: Bug | Status: closed
Component: Database layer | Version: 1.4
(models, ORM) | Resolution: fixed
Severity: Release blocker | Triage Stage: Accepted
Keywords: | Needs documentation: 0
Has patch: 1 | Patch needs improvement: 0
Needs tests: 0 | UI/UX: 0
Easy pickings: 0 |
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Changes (by Kronuz):
* cc: Kronuz (added)
Comment:
I would definitely go for an `annotate()`, for the akaariai's example:
`People.objects.filter(friends__age__gt=F('friends__age') * 2)`
This, after the fix, makes just one join.
If one, however, wants the friends with 2x the age (two separate joins),
it'd need `annotate()` , like so:
`People.objects.annotate(twice_age=F('friends__age') *
2).filter(friends__age__gt=F('twice_age'))`
How far is django from such behavior? ...and also, it doesn't still feel
entirely good (annotate always creating new joins for such cases), since
one might actually want to get the reused join... I would probably suggest
the order of the calling of annotate to change things (if putting annotate
after the filter, it'd reuse the joins from the previous filter, otherwise
it would start afresh). This way, the next query would be equivalent to
the first one:
`People.objects.filter(friends__age__gt=F('twice_age')).annotate(twice_age=F('friends__age')
* 2)`
Provided one could use `F()` of non-yet "annotated" things.
Any thoughts regarding this?
--
Ticket URL: <https://code.djangoproject.com/ticket/18375#comment:22>
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