Transparently implementing an autoincrement trigger?

Question

I'm developing an application using object-relational mapping framework (Toplink Essentials).
The schema design is restricted by certain policy rules, one of which is that every table should have a single primary key column. This also applies to join tables.
I'm having problems figuring out how to make the mapping framework work with this sort of database schema so I thought I'd try to create a PL/SQL trigger which would assign ID's without the mapping framework knowing anything about it.
Now, I've ran into a small problem implementing the trigger: When I leave out the primary key value from INSERT statements I get the following error:
”ORA-00947: not enough values”
Is it possible to create a trigger which would assign the primary key so that the primary key value could be missing entirely from the INSERT statements?
Given the following table definition:
CREATE TABLE myjointable (
primary_key VARCHAR2(18) NOT NULL PRIMARY KEY,
foo_fkey VARCHAR2(18) NOT NULL,
bar_fkey VARCHAR2(18) NOT NULL,
CONSTRAINT uk1 UNIQUE (foo_fkey, bar_fkey)
can I add rows like this:
INSERT INTO myjointable VALUES ('a', 'b');
I've tried it with the following trigger procedure but then I get the "missing values" error:
CREATE OR REPLACE TRIGGER t1
BEFORE INSERT ON myjointable
FOR EACH ROW
BEGIN
IF INSERTING THEN
IF :new.primary_key IS NULL THEN
-- primary_key values are fetched from the szkeygenerate PL/SQL function
SELECT szkeygenerate INTO :new.primary_key FROM dual;
END IF;
END IF;
END;
/

Answer 1

You could hide the table using a view:
SQL> create table t
  2  (x int primary key
  3  ,n varchar2(25)
  4  ,v varchar2(25)
  5  );
Table created.
SQL> create sequence seq
  2  /
Sequence created.
SQL>
SQL> create trigger trg
  2  before insert on t
  3  for each row
  4  begin
  5     if :new.x is null
  6     then
  7        select seq.nextval
  8          into :new.x
  9          from dual;
10     end if;
11  end;
12  /
Trigger created.
SQL> create view v
  2  as
  3  select n, v
  4    from t
  5  /
View created.
SQL> insert into v
  2  values ('test', 'one two');
1 row created.
SQL>
SQL> select *
  2    from t
  3  /
         X N                         V
         1 test                      one two
SQL>