Is_Unique validation with custom db table

[eluser]CroNiX[/eluser]
You should really look into normalized databases and what it means. The way you are going about this will only cause problems and a LOT more work in the end.

You should be using a single table for:
-users
-invoices
-suppliers

The invoices and suppliers tables would also have an index of the user id they belong to. Data wouldn't get mixed. You can have many invoices belonging to a single user and the same with suppliers. This is the proper way to work with relational databases and there is tremendous advantage to doing it this way. I'm sure anyone who has been using databases for a long time, or who has had any database classes, will agree.