Custom validation in model yii

<?php    
  public function rules()
    {

        return array(

            array('roleid', 'verificarCodigo'),
      ); 
    }
    
    
  
    public function verifyCupos($attribute_name,$params)
    {
        if(!isset( $_POST['modelname']) ){
            return true;
        }
        $q = "
            seleccionar codigo que quieras
            ";

        $alr = Yii::app()->db->createCommand($q)->queryAll();
        if(   isset( $cp[0]['cupos'] ) &&  $cp[0]['cupos'] > 0 &&  $cp[0]['cupos'] <= $alr[0]['total'] ){
            $this->addError('userid',
                'El codigo : '.$this->context->course->fullname.' ya paila ' );

            return false;
        } 
            return true;
    }  
    
?>