When we should use Trigger.Old

Understanding Trigger.Old in Salesforce Apex Triggers

Trigger.Old is a context variable available in Salesforce Apex triggers. It provides a read-only list of the original versions of sObject records before they were changed by a DML operation. Knowing when and how to use Trigger.Old is essential for building robust, data-consistent triggers.

When to use Trigger.Old

Use Trigger.Old in these scenarios:

• Before Update or After Update triggers: When you need to compare old and new values to detect changes. For example, tracking field changes, enforcing business rules triggered by specific field updates, or auditing.
• Before Delete or After Delete triggers: To access the record values that are being deleted (since the current record is no longer available after deletion).
• Cascading updates or related record handling: When changes to one record require updates to related records, comparing old and new values helps determine if related updates are necessary.
• Preventing or allowing processing based on changes: Use Trigger.Old to check whether a particular field’s value changed and then decide whether to proceed with additional processing.

Examples

1. Detecting a field change in a Before Update trigger:

for (Integer i = 0; i < Trigger.new.size(); i++) { Account newAcc = (Account)Trigger.new[i]; Account oldAcc = (Account)Trigger.old[i]; if (newAcc.AnnualRevenue != oldAcc.AnnualRevenue) { // Handle revenue change logic } }

2. Using Trigger.Old in a Before Delete trigger to archive values:

for (Account acc : Trigger.old) { // Copy values to a custom archive object before deletion Account_Archive__c arch = new Account_Archive__c(); arch.Name__c = acc.Name; arch.Revenue__c = acc.AnnualRevenue; inserts.add(arch); }

Important considerations

• Trigger.Old is only available in update and delete contexts — it is not available in insert triggers.
• The lists Trigger.new and Trigger.old correspond by index, so use the same index when comparing records.
• Trigger.Old is read-only. To change data, modify Trigger.new records (for before triggers) or perform DML operations on other sObjects.
• Watch for bulk operations — always write triggers that handle collections efficiently to avoid governor limits.

Best practices

• Use Maps keyed by Id when comparing related records to improve performance in bulk operations.
• Keep trigger logic minimal by delegating complex processing to handler classes.
• Avoid SOQL or DML inside loops — bulkify your code.
• Write unit tests that simulate update and delete contexts to verify Trigger.Old behavior.

Using Trigger.Old correctly helps you build predictable, maintainable Apex triggers that respond accurately to record changes.