Rollup Summar trigger for a Lookup Relationship in Salesforce

      Here is the sample Roll-up Summary calculation trigger for Lookup relationship. The below trigger do a Sum Operation using Aggregate Query.

Parent: Account

Child: Contact

Logic: Update parent record field called Total__c with Sum of all related child records Amount__c field value.

1. trigger TotalAmountTrigger on Contact (after insert, after update, after delete, after undelete)
2. {
3.     Set<Id> accountIds = new Set<Id>();
4.     for(contact c: Trigger.isDelete? Trigger.old : Trigger.new){
5.         if(c.AccountId != null)
6.         {
7.             accountIds.add(c.AccountId);
8.         }
9.     }  
10.     List<Account> accList = new List<Account>();
11.     for(AggregateResult currentAggResult:[SELECT AccountId accId, SUM(Amount__c) sumAmt FROM Contact  WHERE AccountId in:accountIds GROUP BY AccountId])
12.     {
13.         Account acc = new Account();
14.         acc.Id = (Id)currentAggResult.get('accId');
15.         acc.Total__c = (Decimal)currentAggResult.get('sumAmt');
16.         accList.add(acc);
17.         accountIds.remove((Id)currentAggResult.get('accId'));
18.     }
19.    
20.     for(Id currAccId : accountIds)
21.     {
22.         Account acc = new Account();
23.         acc.Id = currAccId;
24.         acc.Total__c = null;
25.         accList.add(acc);
26.     }
27.     update accList;
28. }