Unique and null

[Mike Dewhirst]

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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[Derek]

Are you using MySQL?

https://stackoverflow.com/questions/3712222/does-mysql-ignore-null-values-on-unique-constraints

[Ryan Nowakowski]

I'm not sure about the case with null. However, I believe that most uses of CharField leave null=False and use the empty string instead to represent "empty" value.

[Vkash Poudel]

I'm using Postgresql 

does CharField unique solves problem of uniqueness ,

can we work with uuid model.

 

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Best Regards,

Bikash Poudel

[Roger Gammans]

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.

[Roger Gammans]

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

[Mike Dewhirst]

No. PostgreSQL.

I checked that post. And googled the answer. Postgres follows the SQL standard and permits multiple nulls in a unique column.

I should have done that instead of asking. I had first thought of it being a Django thing and couldn't find the answer in the docs.

Thanks for responding.

Cheers

Mike

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[Ing.Daniel Bojorge]

I suggest that if you set unique, the field must have value, so null=False.

Blank=True es for form and template, to set required.  Null es for database.

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[Mike Dewhirst]

Many thanks

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[Ryan Nowakowski]

On Wed, Apr 07, 2021 at 03:25:53PM +0100, Roger Gammans wrote:
> 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

I read the documentation a bit differently. My interpretation is that
CharField(null=True) means that both ''(empty string) and None(null) are
valid values. But no implicit conversion from '' to None takes place.

CharField(null=False) means that None(null) isn't a valid value and that
only ''(empty string) is permitted.

[Roger Gammans]

I understand that reading, and the simple answer is we are both right!

See:
https://github.com/django/django/blob/45a58c31e64dbfdecab1178b1d00a3803a90ea2d/django/db/models/fields/__init__.py#L1082

This shows the model field telling the any auto creating form field,
say like in the admin model, which value to prefer 'empty' values.

Since it's not the model field, so any python code which saves '' and
None, directed onto the model will still have those respected as '' and
NULL.

[Ryan Nowakowski]

Whhhhhyyyyyyyy Oracle!!!!!!! *sigh* :)


find django/db/backends -name "features.py" | xargs grep interprets_empty_strings_as_nulls
django/db/backends/oracle/features.py: interprets_empty_strings_as_nulls = True
django/db/backends/base/features.py: interprets_empty_strings_as_nulls = False