Altering the null attribute of a GeneratedField should raise when making migrations

[Jacob Walls]

The migration layer attempts to detect if a GeneratedField has been altered in a SQL-affecting way so that it can throw an exception:

        if modifying_generated_field:
            raise ValueError(
                f"Modifying GeneratedFields is not supported - the field {new_field} "
                "must be removed and re-added with the new definition."
            )

However, altering the null kwarg, e.g. from False to True is not detected by the above check, but does have an incidence on the DDL that gets emitted, so a migration gets emitted that can do wacky things in the database, see ticket:37348#comment:6.

I propose to fix the modifying_generated_field logic so that it fathoms changes in the null attribute and raises.

This will be a breaking change but most likely a welcome one (removing dangerous operations from your migrations).

Rough test:

tests/migrations/test_operations.py

@@ -6646 +6646 @@
             tests.append(
                 ("test_igfc_4", generated_1, generated_3),
             )
+        generated_4 = models.GeneratedField(
+            expression=F("pink") + F("pink") + F("pink"),
+            output_field=models.IntegerField(),
+            db_persist=db_persist,
+            null=True,
+        )
+        tests.append(("test_igfc_5", generated_2, generated_4))
         for app_label, add_field, alter_field in tests:
             project_state = self.set_up_test_model(app_label)
             operations = [

AssertionError: ValueError not raised