Question

Here’s a good one I think. To start off with, I don’t really know what a trigger is, which might become obvious now.

Are triggers good for maintaining backwards compatibility between database schemas?

Say I have a 1-to-many relationship in version 1. In version 2 this has been changed to a many-to-many. Would a properly implemented trigger be able to handle this so as to allow both versions to run on the same database? Is it a good idea?

Answer 1

Yes, you could do this by utilizing a INSTEAD OF trigger for inserts, updates, and deletes on a view for the old version. To make this more clear, let’s consider the following example:

v1:
    A(id int, data varchar(30))
    B(id int, a_id int, data varchar(30))

v2:
    A(id int, data varchar(30))
    B(id int, data varchar(30), data2 varchar(20))
    AB(a_id int, b_id int)

First you would create a view to mimic the old version of the B table. The tricky thing would be deciding on which of the AB relations to show to the old application. One option would be to add a primary_b column of type bit to the AB table. The view below uses this option

CREATE VIEW OldB AS 
    SELECT B.id AS id, AB.a_id AS a_id, B.data AS data,  
    FROM B INNER JOIN AB ON B.id = AB.b_id
    WHERE AB.primary_b = 1

Next you would create triggers on the view to handle inserts and updates.

CREATE TRIGGER InsertTrigger on OldB INSTEAD OF INSERT
AS
BEGIN

INSERT INTO B (id, data) SELECT id, data FROM inserted;
UPDATE AB SET primary_b = 0 WHERE b_id IN (SELECT id FROM inserted);
INSERT INTO AB (a_id, b_id, primary_b) SELECT a_id, id, 1 FROM inserted;

END 
GO

You would do something similar for instead of update, except that you would have to manage the change in A to B links by deleting and adding rows in AB. I haven’t checked this code out for errors yet, but I will in the next hour or so and post an update if needed.