[5.2.x] Fixed #36453 -- Made When.condition resolve with for_save=False.

Value(None, JSONField()) when used in When.condition incorrectly resolved with
for_save=True, resulting in the value being serialized as SQL NULL instead of
JSON null.

Regression in c1fa3fdd040718356e5a3b9a0fe699d73f47a940.

Thanks to Thomas McKay for the report, and to David Sanders and Simon Charettes
for the review.

Co-authored-by: Sarah Boyce <42296566+sarahboyce@users.noreply.github.com>

Backport of 104cbfd44b9eff010daf0ef0e1ce434385855b13 from main.

django/db/models/expressions.py

@@ -1588,6 +1588,17 @@ class When(Expression):
     def set_source_expressions(self, exprs):
         self.condition, self.result = exprs
 
+    def resolve_expression(
+        self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False
+    ):
+        c = super().resolve_expression(query, allow_joins, reuse, summarize, for_save)
+        if for_save and c.condition is not None:
+            # Resolve condition with for_save=False, since it's used as a filter.
+            c.condition = self.condition.resolve_expression(
+                query, allow_joins, reuse, summarize, for_save=False
+            )
+        return c
+
     def get_source_fields(self):
         # We're only interested in the fields of the result expressions.
         return [self.result._output_field_or_none]

docs/releases/5.2.4.txt

@@ -12,3 +12,7 @@ Bugfixes
 * Fixed a regression in Django 5.2.2 where :meth:`HttpRequest.get_preferred_type()
   <django.http.HttpRequest.get_preferred_type>` incorrectly preferred more
   specific media types with a lower quality (:ticket:`36447`).
+
+* Fixed a regression in Django 5.2.3 where ``Value(None, JSONField())`` used in
+  a :class:`~django.db.models.expressions.When` condition was incorrectly
+  serialized as SQL ``NULL`` instead of JSON ``null`` (:ticket:`36453`).

tests/expressions/tests.py

@@ -29,6 +29,7 @@ from django.db.models import (
     FloatField,
     Func,
     IntegerField,
+    JSONField,
     Max,
     Min,
     Model,
@@ -81,6 +82,7 @@ from .models import (
     Company,
     Employee,
     Experiment,
+    JSONFieldModel,
     Manager,
     Number,
     RemoteEmployee,
@@ -365,6 +367,21 @@ class BasicExpressionsTests(TestCase):
             Number.objects.all(), [None, None], lambda n: n.float, ordered=False
         )
 
+    @skipUnlessDBFeature("supports_json_field")
+    def test_update_jsonfield_case_when_key_is_null(self):
+        obj = JSONFieldModel.objects.create(data={"key": None})
+        updated = JSONFieldModel.objects.update(
+            data=Case(
+                When(
+                    data__key=Value(None, JSONField()),
+                    then=Value({"key": "something"}, JSONField()),
+                ),
+            )
+        )
+        self.assertEqual(updated, 1)
+        obj.refresh_from_db()
+        self.assertEqual(obj.data, {"key": "something"})
+
     def test_filter_with_join(self):
         # F Expressions can also span joins
         Company.objects.update(point_of_contact=F("ceo"))