MYSQL - empty string == 0 problem (updates row with primary key 0 instead of insert) -

actually worked , have no idea i've changed run problem now. tried everything.

the case:
have following query. variable $db-id empty string if content want save new. field module_id primary key. happens query updates row module_id => 0.

the problem
passing empty variable primary key updates row 0 instead of inserting new one.

$this->db->query("insert   modules_text                                  (                                    module_id,                                 module_content,                                 module_page_idx,                                  module_post_id                                 )                   values        (                                 '{$db_id}',                                 '{$content['text']}',                                 '{$content['idx']}',                                 '{$post_id}'                                 )                   on duplicate key                   update        module_content  = '{$content['text']}',                                 module_page_idx = '{$content['idx']}'                 "); 

does have idea how can tell mysql create new row??? appreciated!!!! thank much!!!

saludos sacha!

you may need set primary key field auto increment. example below.

create table tablename (  id mediumint not null auto_increment,  name char(30) not null,  primary key (id))