Fixed #34754 -- Fixed JSONField check constraints validation on NULL values.

The __isnull lookup of JSONField must special case
Value(None, JSONField()) left-hand-side in order to be coherent with
its convoluted null handling.

Since psycopg>=3 offers no way to pass a NULL::jsonb the issue is
resolved by optimizing IsNull(Value(None), True | False) to
True | False.

Regression in 5c23d9f0c32f166c81ecb6f3f01d5077a6084318.

Thanks Alexandre Collet for the report.

django/db/models/lookups.py

@@ -607,6 +607,11 @@ class IsNull(BuiltinLookup):
             raise ValueError(
                 "The QuerySet value for an isnull lookup must be True or False."
             )
+        if isinstance(self.lhs, Value) and self.lhs.value is None:
+            if self.rhs:
+                raise FullResultSet
+            else:
+                raise EmptyResultSet
         sql, params = self.process_lhs(compiler, connection)
         if self.rhs:
             return "%s IS NULL" % sql, params

docs/releases/4.2.5.txt

@@ -9,4 +9,6 @@ Django 4.2.5 fixes several bugs in 4.2.4.
 Bugfixes
 ========
 
-* ...
+* Fixed a regression in Django 4.2 that caused an incorrect validation of
+  ``CheckConstraints`` on ``__isnull`` lookups against ``JSONField``
+  (:ticket:`34754`).

tests/constraints/models.py

@@ -121,3 +121,10 @@ class AbstractModel(models.Model):
 
 class ChildModel(AbstractModel):
     pass
+
+
+class JSONFieldModel(models.Model):
+    data = models.JSONField(null=True)
+
+    class Meta:
+        required_db_features = {"supports_json_field"}

tests/constraints/tests.py

@@ -13,6 +13,7 @@ from django.utils.deprecation import RemovedInDjango60Warning
 from .models import (
     ChildModel,
     ChildUniqueConstraintProduct,
+    JSONFieldModel,
     Product,
     UniqueConstraintConditionProduct,
     UniqueConstraintDeferrable,
@@ -332,6 +333,25 @@ class CheckConstraintTests(TestCase):
         )
         constraint.validate(Product, Product())
 
+    @skipUnlessDBFeature("supports_json_field")
+    def test_validate_nullable_jsonfield(self):
+        is_null_constraint = models.CheckConstraint(
+            check=models.Q(data__isnull=True),
+            name="nullable_data",
+        )
+        is_not_null_constraint = models.CheckConstraint(
+            check=models.Q(data__isnull=False),
+            name="nullable_data",
+        )
+        is_null_constraint.validate(JSONFieldModel, JSONFieldModel(data=None))
+        msg = f"Constraint “{is_null_constraint.name}” is violated."
+        with self.assertRaisesMessage(ValidationError, msg):
+            is_null_constraint.validate(JSONFieldModel, JSONFieldModel(data={}))
+        msg = f"Constraint “{is_not_null_constraint.name}” is violated."
+        with self.assertRaisesMessage(ValidationError, msg):
+            is_not_null_constraint.validate(JSONFieldModel, JSONFieldModel(data=None))
+        is_not_null_constraint.validate(JSONFieldModel, JSONFieldModel(data={}))
+
 
 class UniqueConstraintTests(TestCase):
     @classmethod