Regression on json null value constraints in django 4.2

[Olivier Tabone]

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.