Right now, I'm reading yii-blog-1.1rc.pdf
On page 19, I can't method find that belong to User::model or its parent classes.. I'm confused

<?php class UserIdentity extends CUserIdentity { private $ id; public function authenticate() { $username=strtolower($this->username); $user=User::model()->find('LOWER(username)=?',array($username)); if($user===null) $this->errorCode=self::ERROR USERNAME INVALID; else if(md5($this->password)!==$user->password)