Magento:: Collection Join custom table

protected function _prepareCollection() {
    $collection = Mage::getModel('warranty/warranty')->getCollection()
		->addFieldToFilter('warranty_type', 'New');
    $collection->getSelect()
                ->joinLeft(Mage::getConfig()->getTablePrefix().'warranty_sticker_id_assignment',  'main_table.po_number ='.Mage::getConfig()->getTablePrefix().'warranty_sticker_id_assignment.po_number', array('batch_number','roll_number'));
					
			
    $this->setCollection($collection);
    return parent::_prepareCollection();
}