#34956 closed Cleanup/optimization (wontfix)
Migration should not be allowed for a non-deterministic GeneratedField
Reported by: | Sarah Boyce | Owned by: | nobody |
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Component: | Database layer (models, ORM) | Version: | 5.0 |
Severity: | Normal | Keywords: | |
Cc: | Triage Stage: | Unreviewed | |
Has patch: | no | Needs documentation: | no |
Needs tests: | no | Patch needs improvement: | no |
Easy pickings: | no | UI/UX: | no |
Description
Hello hello
I didn't fully understand the GeneratedField and thought I would be able to do something with comparisons against Now (which I now know I cannot do).
from django.db import models from django.db.models import Case, When, Value, Q from django.db.models.functions import Now class Question(models.Model): question_text = models.CharField(max_length=200) pub_date = models.DateTimeField("date published") closed_date = models.DateTimeField("date closed", null=True) status = models.GeneratedField( expression=Case( When(pub_date__lt=Now(), then=Value("unpublished")), When(Q(closed_date__isnull=False, closed_date__gt=Now()), then=Value("closed")), default=Value("active"), ), db_persist=False, output_field=models.TextField(), )
Django allowed me to make migrations and migrate here (I'm using SQLite) but then when I tried to create a Question object it got very angry.
The traceback is very long but the main thing is:
OperationalError: non-deterministic use of strftime() in a generated column
I learnt my lesson and won't do it again! But I'm wondering if we can bring this error earlier and prevent the user from being able to makemigrations/migrate? I can provide more details if needed
Change History (2)
comment:1 by , 10 months ago
Component: | Uncategorized → Database layer (models, ORM) |
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Resolution: | → wontfix |
Status: | new → closed |
Type: | Uncategorized → Cleanup/optimization |
comment:2 by , 10 months ago
I know the exact same problem was discussed for indexes in the past (I just can't find where). An idea that as brought up was to flag expressions on whether they are deterministic or not but as Mariusz pointed out it's a lot of work to get right.
There are many functions that cannot be used of various databases, e.g.
Random
,SHA512
,SHA384
,SHA224
,SHA1
,MD5
cannot be used on Oracle. As far as I'm, is not feasible for us to curate the list of functions supported for each of database. This is also documented inDatabase limitations
.