When i click list view  button -selected records custom field are updated in corresponding records..Usong onclick javascript

My code :

{!REQUIRESCRIPT("/soap/ajax/19.0/connection.js")} //adds the proper code for inclusion of AJAX toolkit 
var url = parent.location.href; //string for the URL of the current page 
var records = {!GETRECORDIDS($ObjectType.Training__c)}; //grabs the Lead records that the user is requesting to update 
var updateRecords = []; //array for holding records that this code will ultimately update 

if (records[0] == null) {  

//if the button was clicked but there was no record selected 
alert

("Please select at least one record to update."); //alert the user that they didn't make a selection 
} else { //otherwise, there was a record selection 
for (var a=0; a<records.length; a++) { //for all records 
var update_Train = new sforce.SObject("Training__c"); //create a new sObject for storing updated record details 
update_Train.Id = records[a]; //set the Id of the selected Lead record 
update_Train.Training_Status__c= 'Pass'; or  update_Train.Status__c = true;
updateRecords.push(update_Train); 
} 
result = sforce.connection.update(updateRecords); 
parent.location.href = url; //refresh the page 
}

Above code doesnt update record .When i click Please give ur idea when click  list button update or insert  the custom field - please help me.