#35376: Prefetched data not used when combining prefetch_related() and only()
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Reporter: Michael Schwarz | Owner: nobody
Type: New feature | Status: closed
Component: Database layer | Version: 4.2
(models, ORM) |
Severity: Normal | Resolution: duplicate
Keywords: | Triage Stage:
| 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 Simon Charette):
Ah I think this is actually a duplicate of #35044 which is only resolved
in `main`.
What is happening here is that in the third case `restaurant` is
prefetched and stored on the instance but then accessing `city_id` to
prefetch the right `city` requires calling `refresh_from_db` (as it's
excluded from the select mask) then #35044 (which clears all prefetched
data on each `refresh_from_db` call) is triggered and the prefetched
`restaurant` instance is discarded causing the access to the property in
the test to refetch from the database.
The only solution in 4.2 is to include `city` in your select mask (like
you'd want to do anyway to avoid a N+1) to avoid the bugged
`refresh_from_db` call.
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Ticket URL: <https://code.djangoproject.com/ticket/35376#comment:3>
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