Codeigniter Multi database

We are looking for someone to program part of web application In Codeigniter. A master/slave database. Master database can have the users database information, so that at login the user is automatically switched to the slave database. So user information is setup on the master database. In users records each users can be link to a db group A, B, C etc. So using single codeigniter application users are swithched to a corresponding database.