Check constraint validation with NULLs

[David Sanders]

Hi folks,

I've noticed a difference in the way check constraint validation behaves compared to the way Postgres (assuming similar behaviour in other DBs) validates when dealing with NULLs.

Given a model with a simple positive check constraint:

class Test(Model):
    test = IntegerField(null=True)

    class Meta:
        constraints = [
           CheckConstraint(check=Q(test__gte=0), name="positive"),
        ]

Then the an instance with a NULL will save but fail validation:

test = Test()

test.save()

test.validate_constraints()

...
E           django.core.exceptions.ValidationError: {'__all__': ['Constraint “positive” is violated.']}

I believe this is due to the way validation is handled in the WHERE clause. I've trawled back through the PR discussion and original ticket as well as the validation code – I could see the original comment about moving validation to WHERE to get around issues with using SELECT but couldn't find any other mention about how NULLs should be handled.

I wanted to check with folks here first before raising an issue. I'm not sure what the fix should be but I'd recommend at least adding a note in the docs if it's not fixable? :)

Cheers,

David

[Carlton Gibson]

Hi David. 

This is https://code.djangoproject.com/ticket/33996 I think — it's a Release Blocker for 4.1.2 

Your thoughts/review on work there would be welcome. :)

Kind Regards,

Carlton

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