#30564: Cannot create custom field that returns a queryset AND uses pre_save().
-------------------------------------+-------------------------------------
Reporter: Dan J Strohl | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution:
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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Changes (by Dan J Strohl):
* status: closed => new
* resolution: invalid =>
Comment:
Re-opening this, I think you mis-understood my point. I understand that
there are other approaches I can take, and I am looking at them (and am
happy for the suggestions). however, the issue as I reported it actually
IS with Django. The issue is that by using ```if hasattr(value,
'resolve_expression')```, you cannot deterministically tell the difference
between a SQL expression and a Django QuerySet. perhaps there is a way
around this that I haven't seen, in which case, I apologize for missing
it. however by following the docs, when I return a QuerySet object, and
when a pre_save method is used, the system raises an error since the
QuerySet object matches your check, but is not an expression object. (or,
if you ARE considering it an expression object for some reason, then the
bug is that it does not have all of the OTHER required methods that the
expected one does.).
I understand that using ```if hasattr``` is the preferred approach, and I
am not saying that the only fix is to change that, you could so a second
check after it, change the method name in the queryset, have an
alternative path that doesn't check for the method... etc... (all of which
seem like much more work than changing the "if" check), but there may be
other issues that I don't see (I haven't looked deeply enough at the code
to see what other issues that would cause). I don't know the flow or code
well enough to say either way. I simply brought it up as a potential
approach.
Either way though, the bug I am reporting is that when returning a django
queryset, via a custom field with a pre_save, it raises an AttributeError.
Since these are all normal Django objects, and I'm using documented django
approaches, I see this as a bug. If the returned object was something I
controlled, I would simply re-name the "resolve_expression" method so it
didn't' get caught, but I can't (easily) do that for a django QuerySet.
I won't argue this again if you want to close this again, I'm not trying
to get into a war here or anything. I think it's pretty much a corner
case, but I do see it as a bug and wanted to make sure I was at least
being clear.
--
Ticket URL: <https://code.djangoproject.com/ticket/30564#comment:3>
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