Userdata and session data into database

[eluser]someone [/eluser]
[quote author="CroNiX" date="1342459942"]$this->userdata(‘username’);

You're not accessing the session class here...[/quote]

Then I get this error: "Fatal error: Call to undefined method User::userdata()". But if I leave $this->session->userdata(‘username’); in other function in this controller, then it works normally. Error is only when I want to display data from userdata() or compare it to some string or int.

Where am I doing mistake?