code: func (db *Database) Login(user, pass string) error {
  	return db.Session.Login(&Credential{Username: user, Password: pass, Source: db.Name})
  }
docs: // Login authenticates with MongoDB using the provided credential.  The
  // authentication is valid for the whole session and will stay valid until
  // Logout is explicitly called for the same database, or the session is
  // closed.