#35779: ORM to avoid deferring the reference_id of a prefetch
-------------------------------------+-------------------------------------
Reporter: Thiago Bellini | Type:
Ribeiro | Uncategorized
Status: new | Component: Database
| layer (models, ORM)
Version: 4.2 | Severity: Normal
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Suppose that I have a `User` and an `Email` models, and they are related
through `Email.user_id`
If I query `User` like this:
{{{
User.objects.prefetch_related("email_set", Email.objects.only("email"))
}}}
I'll see n+1 issues either way, as `user_id` was deferred, and the ORM
will need to refetch the object from the database.
Although the fix for this is easy, just changing this to:
{{{
User.objects.prefetch_related("email_set", Email.objects.only("user_id",
"email"))
}}}
It can catch some users off-guard, as it is not so obvious that the ORM
will not do "the correct thing".
Based on that, I was wondering if maybe the ORM could force the
reference_id of a relation to not be deferred when it is used in a
`Prefetch` object, the same way it already does with `pk`. It makes sense
to me at least.
---
On a side note, this caused some unexpected behavior at work today, as we
were fixing `RemovedInDjango50Warnings` and the simple addition of
`chunk_size` to an `.iterator()` with a `Prefetch` caused some
`Model.DoesNotExist` issues.
After digging for a while I found out about this issue, which means our
`prefetch_related` was being thrown into the trash, but also because the
iteration was taking some time to happen (very large table), some related
objects got deleted in the meantime, and when the ORM tried to
`refresh_from_db` to get the related id, it failed with that error.
--
Ticket URL: <https://code.djangoproject.com/ticket/35779>
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