>> alter table testtable
>> add constraint testtable_ref_id foreign key (ref_id)
>> references testtable id);
>
>Here you key up your primary not null key with your ref_id field which
>could be null.
>Besides, above definition looks like a circular reference to me.

It should be correct this way.
A more practicable example:

create table employees
(empl_id int4 not null
primary key,
name char (30),

.......

chief int4);

alter table employees
add constraint empl_chief_id foreign key (chief_id)
references employees emplid);

The reason for this is, that every employee could have a chief, which,
of course, is also an employee.

I think this should work correct.
My problem is, that it doesn't work correct in conjunction with
orderingitems.

Regards
Helmut