Fixed #36806 -- Added system check for null kwarg in GeneratedField.

The null argument has no effect on GeneratedField since the nullability
of the column depends on the database and expression used.

django/db/models/fields/generated.py

@@ -30,6 +30,7 @@ class GeneratedField(Field):
         self.expression = expression
         self.output_field = output_field
         self.db_persist = db_persist
+        self.has_null_arg = "null" in kwargs
         super().__init__(**kwargs)
 
     @cached_property
@@ -82,6 +83,7 @@ class GeneratedField(Field):
             *super().check(**kwargs),
             *self._check_supported(databases),
             *self._check_persistence(databases),
+            *self._check_ignored_options(databases),
         ]
         output_field_clone = self.output_field.clone()
         output_field_clone.model = self.model
@@ -188,6 +190,20 @@ class GeneratedField(Field):
                 )
         return errors
 
+    def _check_ignored_options(self, databases):
+        warnings = []
+
+        if self.has_null_arg:
+            warnings.append(
+                checks.Warning(
+                    "null has no effect on GeneratedField.",
+                    obj=self,
+                    id="fields.W225",
+                )
+            )
+
+        return warnings
+
     def deconstruct(self):
         name, path, args, kwargs = super().deconstruct()
         del kwargs["blank"]

docs/ref/checks.txt

@@ -221,6 +221,7 @@ Model fields
   ``GeneratedField``\s.
 * **fields.E223**: ``GeneratedField.output_field`` has errors: ...
 * **fields.W224**: ``GeneratedField.output_field`` has warnings: ...
+* **fields.W225**: ``null`` has no effect on ``GeneratedField``.
 * **fields.E900**: ``IPAddressField`` has been removed except for support in
   historical migrations.
 * **fields.W900**: ``IPAddressField`` has been deprecated. Support for it

docs/ref/models/fields.txt

@@ -1377,6 +1377,9 @@ materialized view.
     backends that support it (SQLite, PostgreSQL, and Oracle) and marked as
     deferred otherwise.
 
+:attr:`~Field.null` has no effect on ``GeneratedField`` since whether the
+column is nullable depends on the database and expression used.
+
 ``GenericIPAddressField``
 -------------------------
 

tests/invalid_models_tests/test_ordinary_fields.py

@@ -1510,3 +1510,26 @@ class GeneratedFieldTests(TestCase):
             Model._meta.get_field("field").check(databases={"default"}),
             expected_warnings,
         )
+
+    @skipUnlessDBFeature("supports_stored_generated_columns")
+    def test_with_null_argument(self):
+        class Model(models.Model):
+            value = models.IntegerField()
+            field = models.GeneratedField(
+                expression=models.F("value") * 2,
+                output_field=models.IntegerField(),
+                db_persist=True,
+                null=True,
+            )
+
+        expected_warnings = [
+            DjangoWarning(
+                "null has no effect on GeneratedField.",
+                obj=Model._meta.get_field("field"),
+                id="fields.W225",
+            ),
+        ]
+        self.assertEqual(
+            Model._meta.get_field("field").check(databases={"default"}),
+            expected_warnings,
+        )