Count the number of child records on the each parent object
trigger ContactTrigger on Contact (after insert, after update, after delete, after undelete) {
//---> above handling all states which could see a contact added to or removed from an account
//---> on delete we use Trigger.Old, all else, Trigger.new
List<Contact> contacts = Trigger.isDelete ? Trigger.old : Trigger.new;
//---> the Set class rocks for finding the unique values in a list
Set<Id> acctIds = new Set<Id>();
for (Contact c : contacts) {
//yes, you can have a contact without an account
if (c.AccountId != null) {
acctIds.add(c.AccountId);
}
}
List<Account> acctsToRollup = new List<Account>();
//****** Here is the Aggregate query...don't count in loops, let the DB do it for you*****
for (AggregateResult ar : [SELECT AccountId AcctId, Count(id) ContactCount
FROM Contact
WHERE AccountId in: acctIds
GROUP BY AccountId]){
Account a = new Account();
a.Id = (Id) ar.get('AcctId'); //---> handy trick for updates, set the id and update
a.Contact_Count__c = (Integer) ar.get('ContactCount');
acctsToRollup.add(a);
}
//----> probably you'll want to do a little more error handling than this...but this should work.
update acctsToRollup;
}Source: developer.salesforce.com