CASCADE and TRIGGER - Some weird problem

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hi, have a little problem with a trigger and the ON DELETE CASCADE
statement.

i'm working on a db that represents Users and Messages. A message can be
owned by more than a user. if i delete a user all his related objects
are deleted too (ON DELETE CASCADE), but if a message that this user
owns is also owned by another user, it has not to be deleted.

i just put ON DELETE CASCADE statement on foreign keys and thos seem to
work.

then i wrote a trigger to check if each message the user owns is owned
by someone else. if it's not delete it!
- -----------------------------------------------------------
- -----this is the code:

CREATE TABLE OWNER(
_LOGIN TEXT,
PRIMARY KEY(_LOGIN) );

CREATE TABLE MESSAGE(
_MESSAGE_ID TEXT,
PRIMARY KEY(_MESSAGE_ID)
);

CREATE TABLE ACCOUNT(
_INDIRIZZO TEXT UNIQUE,
_LOGIN TEXT,
PRIMARY KEY(_INDIRIZZO,_LOGIN),
FOREIGN KEY(_LOGIN)REFERENCES OWNER(_LOGIN) ON DELETE CASCADE);

CREATE TABLE ACCOUNT_MESSAGE(
_MSG TEXT,
_INDIRIZZO TEXT,
PRIMARY KEY(_MSG,_INDIRIZZO),
FOREIGN KEY(_MSG)REFERENCES MESSAGE(_MESSAGE_ID),
FOREIGN KEY(_INDIRIZZO)REFERENCES ACCOUNT(_INDIRIZZO) ON DELETE CASCADE);

CREATE TABLE FOLDER(
_PATH TEXT,
_OWNER TEXT,
PRIMARY KEY(_PATH),
FOREIGN KEY(_OWNER)REFERENCES OWNER(_LOGIN) ON DELETE CASCADE);

CREATE TABLE MSG_IN_FOLDER(
_MSG TEXT,
_FOLDER TEXT,
PRIMARY KEY(_MSG,_FOLDER),
FOREIGN KEY(_MSG)REFERENCES MESSAGE(_MESSAGE_ID),
FOREIGN KEY(_FOLDER)REFERENCES FOLDER(_PATH) ON DELETE CASCADE);

CREATE FUNCTION check_message() RETURNS trigger AS $check_MESSAGE$
BEGIN
DELETE FROM MESSAGE WHERE _message_id IN ( SELECT _MSG
FROM ACCOUNT_MESSAGE NATURAL JOIN msg_in_FOLDER
WHERE _MSG = OLD._MSG
GROUP BY _MSG
HAVING count(*)=1
);
RAISE NOTICE 'Value of OLD._MSG %', OLD._MSG;
RETURN NULL;
END;
$check_MESSAGE$ LANGUAGE plpgsql;

CREATE TRIGGER check_message AFTER DELETE ON MSG_IN_FOLDER
FOR EACH ROW EXECUTE PROCEDURE check_message();
- -------------------------------------------------

- --and these are some values:

delete from OWNER;
delete from ACCOUNT;
delete from MESSAGE;
delete from ACCOUNT_MESSAGE;
delete from FOLDER;
delete from MSG_IN_FOLDER;

insert into OWNER (_login) values ('anna');
insert into OWNER (_login) values ('paolo');
insert into ACCOUNT values ('anna(at)gmail(dot)com', 'anna');
insert into ACCOUNT values ('paolo(at)gmail(dot)com', 'paolo');
insert into MESSAGE (_message_id) values ('1');
insert into MESSAGE (_message_id) values ('2');
insert into ACCOUNT_MESSAGE values ('1', 'anna(at)gmail(dot)com');
insert into ACCOUNT_MESSAGE values ('1', 'paolo(at)gmail(dot)com');
insert into ACCOUNT_MESSAGE values ('2', 'anna(at)gmail(dot)com');
insert into FOLDER (_path, _OWNER) values ('c', 'anna');
insert into MSG_IN_FOLDER values ('1', 'c');
insert into MSG_IN_FOLDER values ('2', 'c');

select * from MESSAGE;

- ----------

as you see there are 2 messages. message 1 is owned both by 'anna' and
'paolo'. message 2 is owned just by 'anna'.

now what i want is that if i delete user 'anna' just message 2 is deleted.

i guess i've done that with my trigger:

...
CREATE FUNCTION check_message() RETURNS trigger AS $check_MESSAGE$
BEGIN
DELETE FROM MESSAGE WHERE _message_id IN ( SELECT _MSG
FROM ACCOUNT_MESSAGE NATURAL JOIN msg_in_FOLDER
WHERE _MSG = OLD._MSG
GROUP BY _MSG
HAVING count(*)=1
);
RAISE NOTICE 'Value of OLD._MSG %', OLD._MSG;
RETURN NULL;
END;
$check_MESSAGE$ LANGUAGE plpgsql;

CREATE TRIGGER check_message AFTER DELETE ON MSG_IN_FOLDER
FOR EACH ROW EXECUTE PROCEDURE check_message();
...

i'm also sure that values it process are right beacuse of that RAISE
NOTICE 'Value of OLD._MSG %', OLD._MSG;

but something seems to go wrong.
all 'anna' stuff is deleted but not her message 2 !

DELETE FROM owner WHERE _login = 'anna';
SELECT * FROM message;

can someone help me to understand why this is not done?

i don't receive any error message from postgres...

thanks

paolo from italy

sonicaaaa(at)gmail(dot)com
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