Message-ID: <42A576E5.1040100@dltmedia.nl>
Date: Tue, 07 Jun 2005 12:28:53 +0200
From: "M.D.G. Lange" <mlange@dltmedia.nl>
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To: pgsql-sql@postgresql.org
Subject: rule or trigger?
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In order for a "dictionary" system I have created the following system:

tbllanguages
- ID
- name
Primary key ( ID )

tbldictionary
- wordid
- languageid
- value
Primary key ( wordid, languageid)

The idea is to have a word id in several languages, so you only have to 
look up the word id and give the language you would like to get the 
message in and you'll be presented the translation.
So far so good... only wordid is not unique, making it not suitable to 
use it in foreign keys... however I'd like a similar idea:

tblsystemmessages
- ID
- wordid
- pgsqlerrorcode
Primary key ( ID )
"Foreign key" wordid references tbldictionary.wordid

It is not possible to create a constraint Foreign key for "wordid". No 
problem there, but I want to be certain that a given wordid exists in 
tbldictionary.
Would I have to create a "RULE" or a "TRIGGER" to be certain that the 
wordid is existing in tbldictionary in whatever language.

I have the idea that a trigger will not prevent the insertion, or did I 
not read well enough?

Would the following rule do what I want?

CREATE OR REPLACE RULE 'systemmessages_rule' AS ON INSERT TO 
"public.tblsystemmessages"
DO INSTEAD (
    IF count ( SELECT DISTINCT tbldictionary.wordid ) > 0
       INSERT INTO tblsystemmessages ( ID, wordid, pgsqlerrorcode ) 
VALUES ( NEW.ID, NEW.wordid, NEW.pgsqlerrorcode )
    ELSE
       NOTHING
);

But would this not recursively call this rule?

Michiel