Conditional rule on Insert in Postgresql - sql

Question

My question is relative to a postgresql problem I encountered.
I want to modify a database containing a newsgroup-like forum. Its messages describe a tree and are stored in a table as below :
\d messages
Table « public.messages »
Column | Type | Modifiers
-----------+-----------------------------+------------------------------------------------------------------
idmessage | integer | not NULL By default, nextval('messages_idmessage_seq'::regclass)
title | character varying(50) | not NULL
datemsg | timestamp without time zone | By default, ('now'::text)::timestamp without time zone
author | character varying(10) | By default, "current_user"()
text | text | not NULL
idgroup | integer | not NULL
msgfather | integer |
(I translated the output from French, I hope I didn't add misinterpretation)
Title is the message title, datemsg its timestamp, author and text are quite explicit, idgroup is the discussion group identifier where the message is found and msgfather is another message this message anwsers to.
The rule I want to implement on insert is about preventing the user to add an answer to a message with a different group from its father's (if it has one, as a new discussion starts with a new message posted without parent).
Now I have this view:
\d newmessage
View « public.newmessage »
Column | Type | Modifiers
-----------+-----------------------+---------------
idmessage | integer |
title | character varying(50) |
text | text |
idgroup | integer |
msgfather | integer |
View definition :
SELECT messages.idmessage, messages.title, messages.text, messages.idgroup, messages.msgfather
FROM messages;
Rules :
ins_message AS
ON INSERT TO newmessage DO INSTEAD INSERT INTO messages (title, text, idgroup, msgfather)
VALUES (new.title, new.text, new.idgroup, new.msgfather)
I can't change view or table as people already use the database, so I think I have to play with rules or triggers.
I tried to add to the view this rule below, without the expected effect:
CREATE OR REPLACE RULE ins_message_answer AS ON INSERT TO newmessage WHERE NEW.msgfather IS NOT NULL DO INSTEAD INSERT INTO messages( title, text, idgroup, msgfather) VALUES ( NEW.title, NEW.text, (SELECT idgroup FROM messages WHERE idmessage=new.msgfather), NEW.msgfather);
Even with this new rule, people can still answer a message and set this answer in another group.
I tried too to change the ins_message rule by adding WHERE NEW.msgfather IS NULL, but I then have the error that no rule is found as on insert in newmessage view when I try to add a message.
So how to achieve what I want to do? With a trigger? (I don't know how to do one with postgresql).

Answer 1

First create an unique index for (idmessage, idgroup):
alter table messages add constraint messages_idmessage_idgroup_key
unique (idmessage, idgroup);
This constraint is needed for msgfather_idgroup_fkey below
as foreign key constraints require matching unique index and
without it there'll be an ERROR: there is no unique constraint matching given keys for referenced table "messages".
Then add self referencing foreign key:
alter table messages add constraint msgfather_idgroup_fkey
foreign key (msgfather,idgroup) references messages(idmessage,idgroup);
In passing:
add NOT NULL to datemsg and author columns;
never use rules.