Trigger to count number of contacts associated with an account ( Bulkified & Uses Aggregate results)

I was working on the similer requirement where I need to check the number of records on the lookup relationship child object and update a field on the parent.

During my search on aggregate results, I found one result which was very easy and thought I will share that here.

Link of the discussion : https://developer.salesforce.com/forums/?id=906F0000000AcdaIAC#ext-gen52

Answered by @Peter_sfdc

	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;
	//---&gt; 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'); //---&gt; handy trick for updates, set the id and update
	a.Contact_Count__c = (Integer) ar.get('ContactCount');
	acctsToRollup.add(a);
	}
	//----&gt; probably you'll want to do a little more error handling than this...but this should work.
	update acctsToRollup;
	}

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