Validation for Field in Form level

Form->DS->Field->Validate()
public boolean validate()
{
boolean ret;

ret = super();

ret = Salestable.validateInventSite();

return ret;
}
-->
SalesTable->Methods->UserDefined Method
public boolean validateInventSite()
{
boolean ret = true;
SalesTable st;


if(this.DPT_NumberSequenceGroupId != this.InventSiteId)
{
ret = false;
info("Not allow to change the site");
}

return ret;
}
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