Table Scripting

This sample shows you how to check if the field has been changed from one value to another

 function UpdateRecord()
 {
   if (Values("case_AssignedUserId")+""=="undefined")
     return;  //bad update from crm..or field not changed
   var CaseRec=CRM.CreateQueryObj("SELECT * FROM Cases WITH (NOLOCK) WHERE " + WhereClause);
   CaseRec.SelectSQL();
   if (CaseRec("case_assigneduserid")!=Values("Case_AssignedUserId"))
   {
     ErrorStr="field value has changed from "+CaseRec("case_assigneduserid") +" to "+Values("Case_AssignedUserId");
   }
 }

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Altering CRM dates in Table level scripts

 var mydate = new Date();
 mydate.setDate(mydate.getDate() +7);
 Values("comp_customerstartdate") = mydate.getVarDate();

Ref: https://community.sagecrm.com/partner_community/b/hints_tips_and_tricks/archive/2011/03/07/setting-or-updating-date-fields-using-a-table-level-script.aspx

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Comparing previous values (in cases where you might want to run some code only when a field value has changed)

Within the UpdateRecord method stub you would first query the current record using

 var crec=CRM.FindRecord("agreement",WhereClause);

You would then compare values as in

 if (crec("comp_name")!=Values("comp_name"))...

For date fields you have to create a Date object

 var c_comp_renewaldate = new Date(crec("comp_renewaldate"));
 var v_comp_renewaldate = new Date(Values("comp_renewaldate"));

and then call the getDate() method of the Date object to do the comparison. You cannot compare the date objects as it will never match

 if (c_comp_renewaldate.getDate() != v_comp_renewaldate.getDate())...