How can I abstract building maps from SObject Lists in a good way? [duplicate]

Question

I frequently find I want/need to map a list of SObjects by a value other than it's own Id.

So, I'd like to abstract this process to make it reusable.

For example, I might want to map Contacts by Account Id.

So I've created a method mapBySpecifiedIdField:

    public static Map<Id, Set<SObject>> mapBySpecifiedIdField (List<SObject> homogeneousSObjectList, SObjectField idField) {
    if (homogeneousSObjectList == null || homogeneousSObjectList.isEmpty()) {
        return null;
    }

    Map<Id, Set<SObject>> sObjectSetBySpecifiedIdMap = new Map<Id, Set<SObject>>();
    for (SObject sObj : homogeneousSObjectList) {
        Id specifiedId = (Id) sObj.get(idField);
        if (!String.isBlank(specifiedId)) {
            if (!sObjectSetBySpecifiedIdMap.containsKey(sObj.Id)) {
                sObjectSetBySpecifiedIdMap.put(sObj.Id, new Set<SObject>());
            }
            sObjectSetBySpecifiedIdMap.get(sObj.id).add(sObj); 
        }
    }

    String mapType = 'Map<Id, Set<' + homogeneousSObjectList[0].getSObjectType() + '>>';
    Map<Id, Set<SObject>> concreteSObjectSetByIdMap = (Map<Id, Set<SObject>>)Type.forName(mapType).newInstance();
    concreteSObjectSetByIdMap.putAll(sObjectSetBySpecifiedIdMap);

    return concreteSObjectSetByIdMap;
}

I'd prefer to invoke it with:

Map<Id, Set<Contact>> mapContactSetByAccountId = (Map<Id, Set<Contact>>) MapHelper.mapBySpecifiedIdField(contactList, Contact.AccountId);

But first, I'll get a compile error:

Incompatible types since an instance of Map<Id,Set<SObject>> is never an instance of Map<Id,Set<Contact>>

If I change my invocation to:

Map<Id, Set<Object>> mapContactSetByAccountId = MapHelper.mapBySpecifiedIdField(contactList, Contact.AccountId);

But this results in a runtime error:

System.TypeException: Invalid conversion from runtime type Map<Id,Set<Contact>> to Map<Id,Set<SObject>>

Is there any way I can make this work?

Answer 1

In this particular case, the issue is with using a Set<SObject> as the value type for your map. I ran into a similar issue (well, one that generated the same error at least) in Is there a specific reason why we can't upcast Sets?

The long and short of it is that things like Set<SObject> test = new Set<Contact>(); don't work because of how Apex is implemented by Salesforce and because of concepts from type theory that I don't fully grasp (covariance, generics, and type erasure).

One workaround in this case is to use a Map<Id, List<SObject>> instead of your Map<Id, Set<SObject>>. Using a List<SObject> here means you lose the property of sets where duplicates are guaranteed not to exist (which doesn't work very well with SObjects in the first place, so I don't think it's that big of a loss).

If you do need to guard against duplicate SObjects, then a Map<Id, Map<Id, SObject>> would be more appropriate (the outer map key would be the Id field that you specify, whereas the inner map key would be the Id field of the SObject being stored).