I need to get the roles of a user from the database.
I want to do it once and before everything. So I call a function from AppController constructor, which uses the session component. The function is given below

public function getRoles() {
    if ($this->Session->check('user_name')) {
        $username = $this->Session->read('user_name');
        $userdesc = $this->Session->read('user_desc');
        if ($userdesc === 'Student') {
            $this->roles['Student'] = true;
        } elseif ($userdesc === 'Faculty') {
            $this->roles['Teacher'] = true;

            $this->loadModel('UserRole');
            $userRolesList = $this->UserRole->getUserRolesList($username);

            $this->loadModel('Role');
            $rolesList = $this->Role->getRolesList();

            foreach($userRolesList as $userRole)
            {
                $this->roles[$rolesList[$userRole['UserRole']['role_id']]] = true;
            }
        }
    }
}

I load session component using an array. But I can't use $this->Session. If I load the component on the fly the function gives other errors. But if I call this function from beforeFilter() it creates no problem. Why I can't do it inside the constructor?

you cant use $this inside constructor as $this is not instanciated yet. explained here PHP: using $this in constructor