Referential Integrity Enforcement

[Cheng Zhang]

When I look closer to the SQL statement generated by django-admin.py  
for MySQL, I realize that it doesn't use 'FOREIGN KEY' clause to  
enforce referential integrity.
For example:
CREATE TABLE `polls_categorys` (
    `id` mediumint(9) unsigned auto_increment NOT NULL PRIMARY KEY,
    `parent_id` integer NULL REFERENCES `tagging_tags` (`id`),
    `child_id` integer NOT NULL REFERENCES `tagging_tags` (`id`)
);
I couldn't find any model-level options to enforce referential  
integrity. Is it intended so that people will have to manually alter  
table to add 'FOREIGN KEY' clause if they want to enforce referential  
integrity?

Thanks.
- Cheng