#25693: Data loss if a ManyToManyField is shadowed by Prefetch
-------------------------------------+-------------------------------------
Reporter: Ian-Foote | Owner: Ian-Foote
Type: Bug | Status: assigned
Component: Database layer | Version: master
(models, ORM) |
Severity: Release blocker | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by aaugustin):
* severity: Normal => Release blocker
* stage: Unreviewed => Accepted
Comment:
I think these are different issues.
----
#21584 demonstrates getting an object, prefetching related objects,
creating an additional related object: the list of prefetched related
objects isn't refreshed automatically. (The description makes it look more
complicated than it actually is.)
That's because parent.children_set is just a queryset. It's consistent
with how querysets have always been working. There is exactly the same
behavior without `prefetech_related`, with any queryset for which the
results have been fetched.
The DRF issue Tom mentions is probably a consequence of this. I believe
DRF should refetch data that may have been invalidated by the updates
before serializing its response.
----
Now, this bug appears to be much worse becase incorrect data gets written
back to the database (if I understand correctly).
Ian, do you know since which version of Django it occurs? I'm wondering if
we can but controls in place to avoid assigning prefetched querysets to
writable descriptors or preventing the write from occurring...
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Ticket URL: <https://code.djangoproject.com/ticket/25693#comment:5>
Django <https://code.djangoproject.com/>
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