#28806: Mechanism of fetching related objects violates READ COMMITTED
assumption of
Django ORM
-------------------------------------+-------------------------------------
Reporter: Aaron Tan | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution:
Keywords: database, read- | Triage Stage:
committed, concurrency control | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by Aaron Tan):
Replying to [comment:1 Tomer Chachamu]:
> The comment is definitely inaccurate, but I don't think the attribute
returning {{{None}}} is the right answer. Sure, the FK is set to NULL on
conflict, but maybe transaction B also updated
{{{Bar.objects.get(pk=<bar's pk>).pk}}} to a new value? In this case, A
will see the new value after a {{{bar.refresh_from_db()}}}, that is not
{{{None}}}.
>
> The docs could probably be improved, to mention that you can use
{{{select_related}}} to prevent this {{{DoesNotExist}}} from being thrown.
(Although, it doesn't remove the fact that you will insert on save, when
you probably meant to update).
I see, although manually modifying primary key seems like a kinda
infrequent operation, but that makes sense. In this case, can we at least
capture {{{ObjectDoesNotExist}}} there and make a partial
`refresh_from_database` by only refreshing and instantiating the related
field in question? Since selector works on a pretty low level of Django's
call stack, it might be possible to do that? After all, this means "any
fetching related object for the first time is theoretically vulnerable to
such concurrency issue", wrapping all of them around {{{try...except...}}}
is cumbersome and error-prone in large and convoluted codebases.
Also, using {{{select_related}}} to supress {{{DoesNotExist}}} was our
first solution, but somehow it does not work. Maybe it is worth
investigating/experimenting.
--
Ticket URL: <https://code.djangoproject.com/ticket/28806#comment:2>
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