How to get Sum of all Order Amounts for an Account?

Question

I'm trying to write an apex code to update a custom currency field Order_Grand_Total__c on my Account Records to display the sum of all orders under that particular Account.

Here's my Account Trigger:

trigger SumOrders on Account (after insert, after update) {

List<Order> orderList = [SELECT id, TotalAmount, AccountId FROM ORDER WHERE AccountId IN: Trigger.newMap.keySet()];

    for (Account a : Trigger.new)
    {  
        for (Order o : orderList)
        {
            if (a.id == o.AccountId && orderList.size() > 0)
            {
                a.Order_Grand_Total__c += o.TotalAmount;
            }
        }
    }
}

And my Test class, which is only covering 57% of the code (4/7):

@IsTest
public class SumOrders {

static testmethod void insertAccount() {

    Account a = new Account();

    a.Name = 'AJ Kreuer2';
    a.Last_Name__c = 'Kreuer2';
    a.Email__c = '[email protected]'; 

    insert a;

}

static testmethod void insertOrder() {

Order o = new Order();

    o.AccountId = '0013B000004Llt6';
    o.Order_ID__c = '0001';
    o.Status = 'Draft';
    o.EffectiveDate = Date.Today();


    insert o;
}


}

The end result is the custom field remains blank:

I'm a bit new to Salesforce and Apex coding so not really sure where to go from here, any ideas? Or is there a simpler way of achieving this?

EDIT:

I've also tried adding a Rollup Summary with the following settings:

However, the Real Time calculation doesn't seem to work, I have to manually click calculate to get the field to populate / update the current order amount on Account Records

Answer 1

Assuming you didn't want to use a roll up summary (either DLRS as mentioned in the comments, or another method), you should be using an AggregateResult query:

 trigger sumOrders on Account (before update) {
     for(Account record: Trigger.new) {
        record.Order_Grand_Total_Amount__c = null;
     }
     for(AggregateResult result: [SELECT SUM(TotalAmount) Amt, AccountId Id FROM Order WHERE AccountId IN :Trigger.new GROUP BY AccountId]) {
         Trigger.newMap.get((Id)result.get('Id')).Order_Grand_Total_Amount__c = (Decimal)result.get('Amt');
     }
 }

At that point, your unit test simply needs to insert an order. Note that for your summary to work correctly, you'll also need to update the accounts from the order trigger:

trigger rollupOrder on Order (after insert, after update, after delete, after undelete) {
    Set<Id> accountIds = new Set<Id>();
    if(Trigger.old != null) {
        for(Order record: Trigger.old) {
            accountIds.add(record.AccountId);
        }
    }
    if(Trigger.new != null) {
        for(Order record: Trigger.new) {
            accountIds.add(record.AccountId);
        }
    }
    update [SELECT Id FROM Account WHERE Id IN :accountIds)];
}

Appropriate error handling is advised (not included here for brevity).