Accessing object reference of a row in a trigger after insert - sql

Question

I have a problem with SQL Object (ORACLE). :(
I have an abstract type Forum, generalized in ForumCategory, and a type Category :
CREATE OR REPLACE TYPE Forum_t AS OBJECT
(
moderators Users_table_ref,
topics Topics_table,
MEMBER FUNCTION getName RETURN VARCHAR2
)
NOT FINAL NOT INSTANTIABLE;
/
CREATE TABLE Forum OF Forum_t
NESTED TABLE moderators STORE AS Liste_moderators,
NESTED TABLE topics STORE AS Liste_topics
(NESTED TABLE posts STORE AS Liste_posts);
CREATE OR REPLACE TYPE ForumCategory_t UNDER Forum_t
(
category REF Category_t,
OVERRIDING MEMBER FUNCTION getName RETURN VARCHAR2
);
/
CREATE OR REPLACE TYPE BODY ForumCategory_t AS
OVERRIDING MEMBER FUNCTION getName RETURN VARCHAR2 IS
c Category_t;
BEGIN
SELECT DEREF(SELF.category) INTO c FROM DUAL;
RETURN c.name;
END getName;
END;
/
CREATE OR REPLACE TYPE Category_t AS OBJECT
(
name VARCHAR2(60)
);
/
CREATE TABLE Category OF Category_t
(
CONSTRAINT PK_CAT_NAME PRIMARY KEY(name)
);
I want to make a trigger after insert on table Category which insert a ForumCategory_t on table Forum.
I have this code :
CREATE OR REPLACE TRIGGER Category_insert
AFTER INSERT ON Category
FOR EACH ROW
DECLARE
ref_cat REF Category_t;
BEGIN
SELECT REF(c) INTO ref_cat
FROM Category c
WHERE c.name = :NEW.name;
INSERT INTO Forum VALUES(ForumCategory_t(Users_table_ref(),Topics_table(),ref_cat));
END;
/
The beginning don't work, I cannot select REF of the inserted Category (it's work outside the trigger).
Please, I need help ^^
PS : Sorry for my english