#28344: Add for_update parameter to Model.refresh_from_db()
-------------------------------------+-------------------------------------
Reporter: Patryk Zawadzki | Owner: (none)
Type: New feature | Status: new
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
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 Aivars Kalvāns):
Replying to [comment:13 Jure Erznožnik]:
> {{{
> with obj.lock_for_update():
> obj.some_attr = 13
> # Would this be auto-saved at the end of the CM's block?
> }}}
>
> However, this one implies both a transaction and an auto-save to me. So
my proposal would be to "decorate" the `lock_for_update()` with three
parameters:
>
> * `atomic: bool=True`
> * `auto-save: bool=True`
> * `refresh_from_db: bool=True`
Can you elaborate on what those parameters would do? If I understand
`refresh_from_db` correctly - it opens the door to the bug mentioned in
comment:10.
{{{
foo = Foo.objects.filter(...).get()
...
Foo.objects.select_for_update.get(pk=foo.pk)
}}}
it leads to a situation where the object `foo` differs from the now locked
database record. That's why I would like to have a single operation to
lock the object and refresh the object with the latest "locked" values
from DB is valuable. I'm all for something like `refresh_from_db` or
`refresh_for_update` to avoid programming errors and reduce number of
database operations at the same time.
That said, I have objections to RoR way of `obj.lock_for_update` or
`obj.with_lock`: it implies that the lock will be released once the block
ends, but that is not the case. Once locked, the record will be locked
until the main transaction is committed or rolled back. Nested
transactions (savepoints) may release locks when rolled back, but it
differs between databases: Oracle does not release locks, but Postgres
does (IMHO). It looks nice, but it gives the wrong impression of how it
works.
--
Ticket URL: <https://code.djangoproject.com/ticket/28344#comment:21>
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