I should have said SQL specifies NULL compares Non-equal, the ORM converts None's in python to NULL in SQL for you, as well as all the other things it does On Wed, 2021-04-07 at 15:25 +0100, Roger Gammans wrote: > Hi All, > Let shed some actually light on the whole problem, before people > start piling in. > This is mostly (form the Django perspective) and issue with > CharFields, because a CharField can store None or ''. For values of > '' in CharField, IIRC, django will convert them to null, with > null=True, blank=True. But not with blank=True,null=False.See also : > https://docs.djangoproject.com/en/3.1/ref/models/fields/#null > So if it stores '', (with null=False) then the uniqueness should work > as expected. > It is should (rather than will), because Django relies on > the backend Database to enforce the constraints, so will depend on > the level of conformance the Backend Db has. > Now SQL Specifies that two variables (or a literal and a field) which > are None will ALWAYS compare as non-equal, and this stackoverflow ( > https://stackoverflow.com/questions/767657/how-do-i-create-a-unique-constraint-that-also-allows-nulls/767702#767702 > ) states the same concept holds for uniqeness . > SQLite, MySQL, Postgres all work, like this, but SQL Server > apparently doesn't. > There's quite a bit there; but I hope it helps and isn't too > confusing. > On Wed, 2021-04-07 at 06:05 -0700, Derek wrote: > > Are you using MySQL? > > https://stackoverflow.com/questions/3712222/does-mysql-ignore-null-values-on-unique-constraints > > > > > > On Wednesday, 7 April 2021 at 08:07:35 UTC+2 Mike Dewhirst wrote: > > > I have just made a CharField unique and Django detects duplicates > > > nicely > > > > > > but seems to ignore multiple instances where the field value is > > > null. > > > > > > > > > > > > arn = models.CharField( > > > > > > max_length=MEDIUM, > > > > > > unique=True, > > > > > > null=True, > > > > > > blank=True, > > > > > > ) > > > > > > > > > > > > Is this correct behaviour? > > > > > > > > > > > > Thanks > > > > > > > > > > > > Mike > > > > > > > > > > > > > > > > > > > > >
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