Summary: In order to add a record into a child table (with a foreign key constrain), Must I specify the child's primary key even though I have a before insert trigger/generator?

If I don't include the child's primary key within the insert statement, an error occurs.



Here are my tables, a parent and a child.

Table: ARTICLE
PRIMARYKEY (primary key constraint)
TITLE

Table: COMMENTS
PRIMARYKEY (primary key constraint)
ARTICLE_FK (foreign key constraint to parent's primary key)
USER_ALIAS
COMMENT

I am able to add child records when I specify its subsiquent primary key ID like this:
INSERT INTO COMMENTS (PRIMARYKEY, ARTICLES_FK, USER_ALIAS, COMMENT) VALUES (36, 1, 'TED', 'HELLO')

I am unable to add child records if I don't specify the primary key ID:
INSERT INTO COMMENTS (ARTICLES_FK, USER_ALIAS, COMMENT) VALUES (1, 'TED', 'HELLO')

Error Message:

Invalid insert or update value(s): object columns are
constrained - no 2 table rows can have duplicate column values.
Violation of PRIMARY or UNIQUE KEY constraint "INTEG_6" on table "COMMENTS".


Here is my child's generator:
CREATE GENERATOR COMMENTS_PRIMARYKEY_GEN;

Here is my child's trigger:
SET TERM ^ ;

CREATE TRIGGER BI_COMMENTS_PRIMARYKEY FOR COMMENTS
ACTIVE BEFORE INSERT
POSITION 1
AS
BEGIN
IF (NEW.PRIMARYKEY IS NULL) THEN
NEW.PRIMARYKEY = GEN_ID(COMMENTS_PRIMARYKEY_GEN, 1);
END^

SET TERM ; ^


Although I have no problem in specifying the child's next primary key, I would really like to know why the child's trigger/generator is not working in this case.