Magento configuration _beforeSave not working on website scope

Question

I am trying to validate my configuration before save but it does not work on website scope.

It works if i choose default config scope.

Here is my code.

 <?xml version="1.0"?>
<config>
<sections>
 <payment>
 <groups>
 <banktransfer>
 <fields>
 <showinfrontend translate="label">
 <label>Enabled On Front-End</label>
 <frontend_type>select</frontend_type>
 <source_model>adminhtml/system_config_source_yesno</source_model>
 <sort_order>1</sort_order>
 <show_in_default>1</show_in_default>
 <show_in_website>1</show_in_website>
 <show_in_store>1</show_in_store>
 </showinfrontend>
 <partpayment translate="label">
 <label>Partpayment Method</label>
 <frontend_type>select</frontend_type>
 <source_model>bankpayment/partpayment</source_model>
 <backend_model>bankpayment/system_config_backend_validatepayment</backend_model>
 <sort_order>1</sort_order>
 <show_in_default>1</show_in_default>
 <show_in_website>1</show_in_website>
 <show_in_store>1</show_in_store>
 </partpayment>
 </fields>
 </banktransfer>
 </groups>
 </payment>
 </sections>
</config>

Model file is as below.

 <?php
class Kgn_Bankpayment_Model_System_Config_Backend_Validatepayment extends Mage_Core_Model_Config_Data
{
 protected function _beforeSave()
 {
 echo "<pre/>";print_r(Mage::app()->getRequest()->getPost());exit;
 $value = $this->getValue();
 $totalpartCollection = Mage::getModel('pdftemplate/totalpart')->getCollection()
 ->addFieldToFilter('payment_id','banktransfer');
 $resource = Mage::getSingleton('core/resource');
 $writeConnection = $resource->getConnection('core_write');
 $table = $resource->getTableName('pdftemplate/totalpart');
 foreach($totalpartCollection as $totalpart){
 $id = $totalpart->getId();
 if($value == $id){
 $query = "UPDATE {$table} SET status = 1,is_default=1 WHERE totalpartrule_id = "
 . (int)$id;
 $writeConnection->query($query);
 }else{
 $query = "UPDATE {$table} SET status = 2,is_default=0 WHERE totalpartrule_id = ". (int)$id;
 $writeConnection->query($query);
 }
 }
 }
}

Answer 1

Got the answer.

But it is strange when i choose website or store view scope and only one of checkbox checked near form field then it is not working if i uncheck all the checkbox and save then it is working. If anyone know how this is actually working please write note here for understand.

Here Is Screenshot