Rails - Mar 2006 - Can not insert null by fixtures

I have the following a table and a fixture(csv file).

create table aaaa (
  id mediumint unsigned not null auto_increment,
  name varchar(192) not null,
  parent_id mediumint unsigned null,
  note varchar(192) not null,
  primary key(id)
)
TYPE=InnoDB;

id,name,parent_id,note
1,Admin Group,,""
2,Guest Group,,""

I expected that null was inserted to parent_id,
but 0 was inserted.

Please help.

Junichi.

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