Question

I’m trying to create a database with history in mind (experience shows you’ll have to do this one day or another).

I’ve asked here database-design-how-to-handle-the-archive-problem but there’s no better anser than the link here.

My problem is about where to do the code and technically, how (MySQL gives me headaches). First I’ve started doing this in Php: before doing any insert, duplicate the record mark it as “obsolete” then modify the record.

But there’s a dependency problem (manytomany and manytoone associations must be updated as well) which implies coding (one way or another) all the dependancies and updates that come with the tables (which is not acceptable).

So I’m thinking about doing all the work on the database server side. This would greatly simplify my Php code.

The problem is that I have to “archive” the current record before modifying it. To do so, the code must be in a trigger “before update”.

Here’s my code:

DELIMITER ;;

DROP TRIGGER IF EXISTS produit_trigger_update_before;

CREATE TRIGGER produit_trigger_update_before
BEFORE UPDATE ON produit
  FOR EACH ROW BEGIN
    /* */
    INSERT INTO produit SET 
      id_origine      = OLD.id_origine,
      date_v_creation = OLD.date_v_creation,
      date_v_start    = OLD.date_v_debut,
      date_v_end      = NOW(),
      ...
      last_record     = OLD.last_record;

    /* Dependancies : */
    SET @last=LAST_INSERT_ID();
    UPDATE categorie_produit SET id_produit=@last
    WHERE id_produit = OLD.id;
    UPDATE produit_attribut SET id_produit=@last
    WHERE id_produit = OLD.id;
  END;;

DELIMITER ;;

If I get this code working, all my problems are gone. But damn it, it’s not working:

mysql> update produit set importance=3;
ERROR 1442 (HY000): Can't update table 'produit' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.
mysql> Bye

In this page there’s a working sample, which uses INSTEAD OF UPDATE clause in the trigger. MySQL doesn’t seem to support this.

So my question is both conceptual (= have you any other “principle” that could work) and/or technical (= can you make this trigger work).

Answer 1

If I get this code working, all my problems are gone. But damn it, it’s not working:

As a rule you can’t have a trigger on table A trigger inserts into table A – since that could cause an endless loop. (Trigger mutation in Oracle terms)

Personally I would not do this using triggers. Triggers can do “audit logging” – but this is not what you want here.

I suggest you solve it programatically – either with a PHP function or a MySQL stored procedure (whatever your preference) that you call something like “ModifyProduit”.

The code would then do basically what you have the trigger above do. (It might be easier to just have the code set date_v_end on the current row, and then insert a completly new row. That way you don’t have to mess around with updating your referenced tables)