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.

[Natalia Bidart]

I think this is related to #34754, and I originally thought that this was another consequence of what Simon said there: I think the fundamental problem here is that:

MyModel.objects.create(values=None)  # Creates an object with a SQL NULL
MyModel.objects.filter(values=None)  # Filters for objects with JSON NULL

but then I ran the proposed test with --debug-sql and noticed that the check code is casting the value in the database to ::JSONB independently of whether the value is None or not:

INSERT INTO "ticket_35381_jsonfieldmodel" ("data")
VALUES (NULL) RETURNING "ticket_35381_jsonfieldmodel"."id";

args=(NONE,);

ALIAS=DEFAULT (0.000)
SELECT 1 AS "_check"
WHERE COALESCE((NOT ('null'::JSONB = 'null'::JSONB)), TRUE);

args=(Int4(1),
      Jsonb(NONE),
      Jsonb(NONE),
      TRUE);

ALIAS=DEFAULT

So I kept digging and bisected the "bad" commit to 5c23d9f0c32f166c81ecb6f3f01d5077a6084318, and something like this makes the test pass:

django/db/models/fields/json.py

@@ -103 +103 @@
             value.output_field, JSONField
         ):
             value = value.value
-        elif hasattr(value, "as_sql"):
+        elif value is None or hasattr(value, "as_sql"):
             return value
         return connection.ops.adapt_json_value(value, self.encoder)

it makes the test model_fields.test_jsonfield.TestSaveLoad.test_json_null_different_from_sql_null fails for the check on the query:

NullableJSONModel.objects.filter(value=Value(None, JSONField())) ==  [json_null]

So further investigation is needed to come up with an acceptable solution.

[Natalia Bidart]

Ticket #35167 is related but I don't think is a dupe.

[Simon Charette]

I've looked into it as well and #35167 only provides a partial solution here IMO as it would require the Q.check logic to also pass around for_save to activate the code path it introduces.

I'm starting to believe that in order to finally solve this null ambiguity problem we should

1. Introduce a JSONNull expression to disambigutate between the two. It would also allow the creation of model instances with a JSON null which is convoluated today as it requires models.Value(None, models.JSONField()) to be used.
2. Deprecate filter(jsonfield=None) meaning JSON null by requiring JSONNull to be used instead. Should we only do this at the top level to still allow jsonfield__key=None to filter against null keys? An alternative would be to introduce a __jsonnull lookup.

The good news is that Django makes it very hard to store JSON null in the first place since #34754 so this kind of constraints should be rarely needed.

[Natalia Bidart]

Thank you Simon. In my view, and in an ideal situation, we shouldn't allow "anywhere" to use None as the json's null. I think we should reserve None only to represent SQL's NULL, and have a representation for the (string) json's null (I understand this matches the new JSONNull expression you are proposing).

Should we only do this at the top level to still allow jsonfield__key=None to filter against null keys?

I would say no, because it then gets very confusing.

An alternative would be to introduce a __jsonnull lookup.

Would this lookup allow further lookups to be chained?

[Simon Charette]

I think we should reserve None only to represent SQL's NULL, and have a representation for the (string) json's null (I understand this matches the new JSONNull expression you are proposing).

Yes it matches the JSONNull proposal. If we take away the ability to use None for filtering against JSON null we should provide an alternative.

The deprecation could look like

1. Introduce JSONNull
2. Make filter(jsonfield=None) raise a deprecation warning pointing at either using filter(jsonfield__isnull=True) or filter(jsonfield=JSONNull)
3. At the end of the deprecation period switch filter(jsonfield=None) to mean filter(jsonfield__isnull=True) like on all other fields

It leaves the problem of having JSON null not surviving a round trip to the database as both SQL NULL and json.loads("null") are turned into Python None but that's a different issue that can be addressed with a specialized decoder if users require it.

Would this lookup allow further lookups to be chained?

It could or not, what gets returned from a lookup can be prevent further chaining if we want.