php - How to use dbforge[of codeigniter] to add foreign key -

I am using codeigniter and I am trying to convert the following query into deborph style query, I How can I do this?

  To create table filter (filter int main key auto_increment, filtename varchar (50) not tap, not classified int tap, passive tinyint zero, sequence int null, foreign key (category) reference Category (id));    
 
 
 Use  $ this-> Db- & gt; Query ();  
 Instead  
 
  $ sql = "Make the filter filter (filter int primary key auto_increment, filtername varchar (50) not tap, not classified int tap, passive tinent faucet, sequence int null, foreign key (Category) reference category (ID) "; $ This- & gt; Db-query ($ sql);