So far, I've created a map of Ultimate Parent Accounts and their Child Accounts (i.e., account hierarchies). Now, for each Ultimate Parent Account, I need to loop through the Child Accounts and get the sum of their Employees. Then, I will update the Total Hierarchy Employees field with that sum for each account in the hierarchy.

One important item to note—I'm not using the standard Parent Account field because I need the logic to be able to handle multiple tiers as well as include accounts that do not have a parent (for example, accounts that are not part of a hierarchy are their own Ultimate Parent Accounts). Also, I can't use AggregateResults because the field I'd be grouping by is a formula field.

Appreciate any assistance here!

public class AccountController {

    public static void countTotalHierarchyEmployees(Account[] accounts) {
    
        List<String> ultimateParentAccountIDs = new List<String>();
        
        for(Account a : accounts) {
        
            if(!ultimateParentAccountIDs.contains(a.Ultimate_Parent_Account_ID__c)) {
                ultimateParentAccountIDs.add(a.Ultimate_Parent_Account_ID__c);
            }
        }
        
        System.debug('ultimateParentAccountIDs List ---> ' + ultimateParentAccountIDs);
        
        List<Account> childrenAccts = [SELECT Id, NumberOfEmployees, Ultimate_Parent_Account_ID__c FROM Account
                                       WHERE Ultimate_Parent_Account_ID__c IN : ultimateParentAccountIDs];
    
        Map<Id, List<Id>> parentToChildrenMap = new Map<Id, List<Id>>();
        
        for(Account a : childrenAccts) {
            if(parentToChildrenMap.containsKey(a.Ultimate_Parent_Account_ID__c)) {
                List<Id> childrenIDs = parentToChildrenMap.get(a.Ultimate_Parent_Account_ID__c);
                childrenIDs.add(a.Id);
                parentToChildrenMap.put(a.Ultimate_Parent_Account_ID__c, childrenIDs);
            }
            else {
                parentToChildrenMap.put(a.Ultimate_Parent_Account_ID__c, new List<Id> { a.Id });
            }
        }
        
        System.debug('parentToChildrenMap ---> ' + parentToChildrenMap);
    }
}