i have magento collection 

```
Mage::getResourceModel('customizebundles/customizebundles_collection')
                            ->distinct(true)->addFieldToSelect('validity')
                            ->setOrder('validity', 'asc');

```

how can i convert `validity` column `varchar` to `unsigned` like

```
SELECT DISTINCT validity FROM customize_bundles AS main_table order by cast(validity as unsigned)

```