Login from user specific databse in mongodb

I am trying to implement user specific database . For this i am trying to login from master database and then i want to switch to another database (ie database by name of user id). after login currently i am able to change database (By setting db name by user name)and when more login from another user-id is their then the database of previous user is changed by new user-id's database name. Can any one help me . Database is Mongo db