Understanding Trigger.old and Trigger.new Salesforce Apex Triggers

Use Case

When a field value is updated to a certain value, use Trigger.old and Trigger.new to compare the older and new version values of field values on a record and perform the required business logic accordingly.

• Trigger.old is available only on update and delete events, and represents the old version of the records before the trigger event.
• Trigger.new is available only on insert, update, and undelete triggers, and represents the new version of the records after the trigger event.

Example Scenario

Consider the following setup: You have a workflow rule on Account object creation that updates a field such as the Description field. You then have a trigger that fires after the workflow update and iterates over Trigger.old, expecting to get the Description field that was modified by the workflow. In this case, you will get a NullPointerException.

Rationale

The reason for the NullPointerException comes down to understanding what values these two context variables hold at different points in the transaction.
Trigger.old values represent the state of records before the CURRENT transaction (the user's action that initiated the trigger). Trigger.new represents the state of records after the current transaction's DML completes.
In the example above, since it is an insert scenario, Trigger.old will NOT hold the Description field updated by the workflow rule — because the workflow fired as part of the same insert transaction and Trigger.old captures the pre-transaction state.

You'll have to query the database after the workflow field update fires In order to obtain that same field.

For example:
 

List<Account> accts = [Select Id, FieldUpdatedByWorkflow__c from Account where Id in : Trigger.old];

See the sample trigger below.

Note: Create a workflow field update on Account objects creation to see the results in the debug log.

trigger testTrigger on Account (before update, after update, before insert, after insert) 
{
    if (Trigger.isBefore) {
        System.debug('********Trigger values***********');
        System.debug('***SFDC: Trigger.old is: ' + Trigger.old);
        System.debug('***SFDC: Trigger.new is: ' + Trigger.new);
    }
    
    if (Trigger.isAfter) {
        System.debug('********Trigger values***********');    
        System.debug('***SFDC: Trigger.old is: ' + Trigger.old);
        System.debug('***SFDC: Trigger.new is: ' + Trigger.new);
       
    }
}

However, Trigger.new WILL contain the Description field updated by the workflow, along with any other fields populated at object creation.

To access the workflow-updated field in Trigger.old, you must perform an explicit SOQL query after the workflow update fires. For example, query the Account object by the record IDs in Trigger.old: SELECT Id, FieldUpdatedByWorkflow__c FROM Account WHERE Id IN :Trigger.old. This retrieves the most current version of those records from the database, including any workflow-updated fields.

The sample trigger provided in the additional resources logs both Trigger.old and Trigger.new during before and after insert and update events on Account. Running this trigger with a workflow field update on Account creation allows you to observe in the debug log how Trigger.new reflects the workflow update while Trigger.old does not during the insert event.