id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
35381	Regression on json null value constraints in django 4.2	Olivier Tabone	nobody	"Regression is still present in 5.0 and main branch

given a model defined as follow

{{{
from django.db import models


class JSONFieldModel(models.Model):
    data = models.JSONField(null=True, blank=True, default=None)

    class Meta:
        required_db_features = {""supports_json_field""}
        constraints = [
            models.CheckConstraint(
                check=~models.Q(data=models.Value(None, models.JSONField())),
                name=""json_data_cant_be_json_null"",
            ),
        ]

}}}

the following test works in django 4.1.13, fails in later version

{{{
from django.test import TestCase, skipUnlessDBFeature

from .models import JSONFieldModel


class CheckConstraintTests(TestCase):
    @skipUnlessDBFeature(""supports_json_field"")
    def test_full_clean_on_null_value(self):
        instance = JSONFieldModel.objects.create(data=None)  # data = SQL Null value
        instance.full_clean()

}}}

{{{
tests/runtests.py json_null_tests                    
======================================================================
ERROR: test_full_clean_on_null_value (json_null_tests.tests.CheckConstraintTests.test_full_clean_on_null_value)
----------------------------------------------------------------------
Traceback (most recent call last):
  File ""/Users/olivier/dev/django.dev/olibook/django/django/test/testcases.py"", line 1428, in skip_wrapper
    return test_func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ""/Users/olivier/dev/django.dev/olibook/django/tests/json_null_tests/tests.py"", line 13, in test_full_clean_on_null_value
    instance.full_clean()
  File ""/Users/olivier/dev/django.dev/olibook/django/django/db/models/base.py"", line 1504, in full_clean
    raise ValidationError(errors)
django.core.exceptions.ValidationError: {'__all__': ['Constraint “json_data_cant_be_json_null” is violated.']}

----------------------------------------------------------------------
}}}

Attached a patch that will create run json_null_tests folder in django's `tests` directories. Test can be run with

{{{
tests/runtests.py json_null_tests
}}}

NOTE : in django 5.1 the `CheckConstraint.check` parameter as been renamed to `CheckConstraint.condition`.

I ran a git bisect on this test case:

8fcb9f1f106cf60d953d88aeaa412cc625c60029 is bad
e14d08cd894e9d91cb5d9f44ba7532c1a223f458 is good

{{{
git bisect run tests/runtests.py json_null_tests
}}}

5c23d9f0c32f166c81ecb6f3f01d5077a6084318 is identified as first bad commit

the issue appears with both sqlite and postgres backends

A few words on what we I'm trying to achieve (some context on the regression):

The `json_data_cant_be_json_null` aims to prevent having `JsonModel.data = None` (SQL Null) and `JSONModel.data = Value(None, JSONField())`  (JSON null) values in the database. These leads to unwanted behaviors when filtering on the null value, and I settled on representing the absence of value with SQL Null and not JSON null.

The app was running in django 4.1 and I'm upgrading it to later django version. That's how the issue appeared.
"	Uncategorized	new	Database layer (models, ORM)	5.0	Normal				Unreviewed	0	0	0	0	0	0