How do I reference the instance in a constraint on an object table?

Question

I want to create a unique constraint that involves the result of a deterministic member function.
I can do it just fine if I create table with a column of the object type:
drop table test_table1;
drop table test_table2;
drop type test_type;
create type test_type as object (
     alpha varchar2(30),
     member function get_beta return varchar2 deterministic
show errors;
create type body test_type as
     member function get_beta return varchar2 deterministic is
     begin
          return 'Beta';
     end;
end;
show errors;
create table test_table1 (
     test_instance mattk.test_type,
     beta varchar2(4000) as (test_type.get_beta(test_instance)),
     constraint test_table1_pk primary key (beta)
insert into test_table1 (test_instance) values (test_type('Alpha'));
commit;
insert into test_table1 (test_instance) values (test_type('Gamma'));
commit;As I said, this works just fine, and the second insert statement produces the following error:
insert into test_table1 (test_instance) values (test_type('Gamma'))
ORA-00001: unique constraint (MATTK.TEST_TABLE1_PK) violatedHowever, I'd really like my table to be an object table.
If I do that:
create table test_table2 of test_type;How do I then reference the object instance that the row represents?

Answer 1

I've got it all worked out now. Thanks everyone for the helpful advice.
I changed the type definition slightly. I added an Attribute for Beta, in addition to keeping the deterministic member function.
drop table test_table2;
drop type test_type;
create type test_type as object (
     alpha varchar2(30),
     beta varchar2(30),
     member function get_beta return varchar2 deterministic
show errors;
create type body test_type as
     member function get_beta return varchar2 deterministic is
     begin
          if (self.beta <> 'Beta') then
               raise_application_error(-20000, 'Beta is not properly set.');
          end if;
          return 'Beta';
     end;
end;
show errors;
create table test_table2 of test_type;
alter table test_table2 add (
     constraint test_table2_pk primary key (alpha, beta)
create unique index test_table2_idx1 on test_table2(test_type.get_beta(object_value));
insert into test_table2 values (test_type('Alpha', 'Beta'));
commit;
insert into test_table2 values (test_type('Gamma', 'Beta'));
update test_table2 t set t.beta = 'BETA';
insert into test_table2 values (test_type('Delta', 'Delta'));Correctly produces the following errors:
insert into test_table2 values (test_type('Gamma', 'Beta'))
ORA-00001: unique constraint (MATTK.TEST_TABLE2_IDX1) violated
update test_table2 t set t.beta = 'BETA'
ORA-20000: Beta is not properly set.
ORA-06512: at "MATTK.TEST_TYPE", line 5
insert into test_table2 values (test_type('Delta', 'Delta'))
ORA-20000: Beta is not properly set.
ORA-06512: at "MATTK.TEST_TYPE", line 5The presence of the unique index over the deterministic member function guarantees that function will be called on any insert or update, which gives it the opportunity to error out if the attribute is not properly set.
The reason I was interested in doing this is because Oracle does not support constant attribute values.
Again, thanks everyone for the helpful replies.