Workflow ISCHANGED() translated to Apex trigger

Workflow is great. I can simply and declaratively make changes, and can easily update things like email templates, criteria, tasks, etc. without using Eclipse and writing/running unit tests.
Sometimes, however, workflow isn’t enough; we need to use a trigger.

Today, I had a use-case that when a DateTime field is filled, a contact (identified via a lookup on a parent object – so a grandparent record) should receive a notification. That’s easy enough to do with a ISCHANGED(DateTimeField__c) workflow criteron, but the Email Alert can’t “find” the contact. A trigger is necessary.

Here’s how I coded the trigger:

trigger InterviewUpdates on Interview__c (after update, before update) {
	if(Trigger.IsBefore){
		for(Interview__c i : Trigger.New){
			DateTime dt = i.Date_and_Time__c;
			if(Trigger.oldMap.get(i.id).Date_and_Time__c != dt){
				// Send email to applicant, passing Interview, DateTime, and Contact 
				// (which we found via a query coded outside the loop to avoid governor limits)
			}
		}
	}
}

I’ve left out a lot of parts here, but I hope that the main bit of using Trigger.oldMap to find the old value and compare it to the new one comes through.

Happy coding!