public function getModel()
    {
        if ($this->_model === null) {
            $this->_model = Nav::findOne($this->navId);
            
            if (empty($this->_model)) {
                throw new Exception('The model active record could not be found for the corresponding nav item. Maybe you have inconsistent Database data.');
            }
        }
        
        return $this->_model;
    }