#36472: GeneratedField(primary_key=True) crashes on create(), and other issues
-------------------------------------+-------------------------------------
Reporter: David Sanders | Owner: (none)
Type: Uncategorized | Status: new
Component: Database layer | Version: dev
(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 Simon Charette):
* cc: Simon Charette (added)
Comment:
I haven't played with it a lot so take this with a grain of salt but I
think the crux of the issue here is that we mark generated field as
deferred on instance that are meant for ''adding''.
We don't do that for any other fields even the one that are db generated
(think of `Field.db_default` usage) and it makes little sense knowing
there's nothing to fetch until persistence is performed.
If we're speaking code here I meant that for a model of the form
{{{#!python
from django.db import models
class Foo(models.Model):
field = models.IntegerField()
field_db_default = models.IntegerField(
db_default=models.Value(1)
)
field_generated = models.GeneratedField(
expression=models.F("field"),
db_persist=True,
output_field=models.IntegerField()
)
def run():
instance = Foo()
print(repr(instance.field))
print(repr(instance.field_db_default))
try:
print(repr(instance.field_generated))
except Exception as exc:
print(f"Failed to access instance.field_generated: {exc}")
}}}
[https://dryorm.xterm.info/ticket-36472 You'd get] the following output
{{{
None
<django.db.models.expressions.DatabaseDefault object at 0x7f9ba68a27b0>
Failed to access instance.field_generated: Cannot read a generated field
from an unsaved model.
}}}
As David pointed out on the forum this has a bit of overlap with #27222.
--
Ticket URL: <https://code.djangoproject.com/ticket/36472#comment:2>
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