sql loader

I am trying to write a control file for a structure the following tables and am failing miserably, can someone help?

drop table a_table;
drop table b_table;
drop type a_type;

create or replace type a_type as object ( a_key int,
a_val int);
/

create table a_table of a_type (a_key primary key ) OBJECT IDENTIFIER IS PRIMARY KEY;
/

create table b_table (
a int,
a_key ref a_type scope is a_table);

a_table is loaded with the following inserts insert into a_table values (a_type(1,2)); insert into a_table values (a_type(2,2));

Control File.

######################################################################

LOAD DATA
INFILE *
into table b_table
append
FIELDS TERMINATED BY ',' TRAILING NULLCOLS (

        a char,
        a_filler filler char,
        a_key ref (constant a_table,a_key,a_filler)

2,1
2,1
2,2