How to handle Roll Up Summary type Calculations WITHOUT using Master-Detail

Question

I currently have an object called Opportunities__c with a child object related to it, Opportunities_Forecast__c via a Master-Detail relationship. The Forecast__c object contains two number fields that ultimately get summed up using a Roll-Up Summary field on the Opportunities__c object.

The nature of the object needs to change now in that Opportunities_Forecast__c is just a general forecasting object and could be related to the Account object or Opportunities__c object, thus it may not always have a relationship to an Opportunity__c record. I know Master-Detail fields are required on child objects, so I have to change the data type to a Lookup.

I can't do this (yet) because of the Roll-Up summary fields. So, I was wondering what the best way to handle the functionality of Roll-Up summary fields, without them being Roll-Up summary fields.

One idea I had was to use a Trigger that would check for Create, Update and Delete events and recalculate the fields accordingly, but I was wondering if there was a better way to do this.

Answer 1

The standard go-to answer is to use Andrew Fawcett's excellent Declarative Rollup Summary package. There's a managed package install link, and it comes complete with a UI for configuring your desired rollups. It works on any object that supports custom fields and triggers, so give it a go! Also, it's open source, so you can borrow bits and pieces if you decide to give it a go yourself. Finally, this has definitely been answered here on SFSE before, including an answer of my own. The actual implementation is pretty trivial when you get down to it, but you do need to pay attention to the details.

Answer 2

You are right, a trigger is the best Solution.

You will need to have a trigger that maps and groups all children by parent. So you can iterate across the parents children. Not tested Psuedo Code:

trigger(After Insert/Update){
    map<id, list<child>> parentChild_Map = new map<id, list<child>>();
    map<id,parent> parent_Map = new map<id,parent>();
    set<id> parent_Set = new set<id>();
    set<id> parent_Set2 = new set<id>();

    for(children c : trigger.new){
        parent_Set.add(c.parentId);
    }
    parent_Map = [SELECT id, sum__c FROM Parent WHERE Id IN parent_Set];
    children_List = [SELECT id, parentid, value__c FROM child WHERE parentId IN parent_Set];

    for(child c : children_List){
        if(parent_Set2.contains(c.parentId)) {
           parent_map.get(p.ParentId).sum__c = parent_map.get(p.ParentId).sum__c + c.value__c;
        else{
           parent_Set2.add(c.ParentId);
           parent_map.get(c.ParentId).sum__c = c.value__c;
        }
    }

    update parent_map.values();
}