Re: [Django] #30564: Cannot create custom field that returns a queryset AND uses pre_save().

[Django]

#30564: Cannot create custom field that returns a queryset AND uses pre_save().
-+-
 Reporter:  Dan J Strohl |Owner:  nobody
 Type:  Bug  |   Status:  closed
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:  invalid
 Keywords:   | Triage Stage:
 |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by felixxm):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 > 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.

 Your custom field implementation is quite complicated. Custom fields with
 `pre_save()` works properly because you can treat any Django's field as a
 custom field e.g. `DateField`. It seems (but it is hard to tell without
 `pre_save_func()`) that in your case `pre_save()` returns `Query` which is
 not expected by `SQLInsertCompiler.prepare_value()`, that's why IMO it is
 an issue in your implementation/approach.

 > Since these are all normal Django objects.

 You cannot assume that Django internals will handle everything, even if
 these objects are defined by Django.

 I think that `return self.get_db_prep_value(value)` in `pre_save()` in
 branch with `pre_save_func()` should fix your issue.

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Re: [Django] #30564: Cannot create custom field that returns a queryset AND uses pre_save().

[Django]

#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
-+-

Comment (by Simon Charette):

 > 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.

 FWIW even if the `QuerySet` class doesn't completely implement the
 expression API it partially behaves like one; that's what makes usage of
 `QuerySet` as subqueries work (e.g.
 `.filter(foo__in=Foo.objects.filter(bar=1))`. In that sense `QuerySet` is
 an SQL expression.

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Ticket URL: 
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Re: [Django] #30564: Cannot create custom field that returns a queryset AND uses pre_save().

[Django]

#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
-+-
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.

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Ticket URL: 
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Re: [Django] #30564: Cannot create custom field that returns a queryset AND uses pre_save(). (was: Cannot create custom field that returns a queryset AND uses pre_save())

[Django]

#30564: Cannot create custom field that returns a queryset AND uses pre_save().
-+-
 Reporter:  Dan J Strohl |Owner:  nobody
 Type:  Bug  |   Status:  closed
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:  invalid
 Keywords:   | Triage Stage:
 |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by felixxm):

 * status:  new => closed
 * version:  2.2 => master
 * resolution:   => invalid


Comment:

 Thanks for the report, however it is not an issue in Django. It seems that
 you want to get help with your custom implementation.

 If you're on PostgreSQL you can use
 [https://docs.djangoproject.com/en/2.2/ref/contrib/postgres/fields/#arrayfield
 ArrayField] without implementing a custom field. If you're on a different
 database and you need to implement a custom field for that, then please
 use one of
 [https://code.djangoproject.com/wiki/TicketClosingReasons/UseSupportChannels
 support channels].

 `if hasattr(value, 'resolve_expression')` is the way that we prefer to
 check if something is an expression, it is used in many places.


 Closing per TicketClosingReasons/UseSupportChannels.

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Ticket URL: 
Django 
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Re: [Django] #30564: Cannot create custom field that returns a queryset AND uses pre_save()

[Django]

#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:  2.2
  (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
-+-

Comment (by Dan J Strohl):

 Perhaps instead of using "hasattr(value, 'resolve_expression')", a
 "issubclass(value, ):" could be used?

-- 
Ticket URL: 
Django 
The Web framework for perfectionists with deadlines.

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