Trigger.old
This variable returns a list of the old version of sObject records. This list is only available in update and delete triggers
trigger ApexTrigger on Opportunity(before update) {
// Only available in Update and Delete Triggers
for (Opportunity oldOpp: Trigger.old) {
for (Opportunity newOpp: Trigger.new) {
if (oldOpp.Id == newOpp.Id && oldOpp.Amount != newOpp.Amount) newOpp.Amount.addError('Amount cannot be changed'); // Trigger Exception
}
}
}
