Test case for deletion of unneeded (childless) parent objects

gistfile1.java

trigger UpdateRelatedObjects on Contact (after update) {
    // this is meant for reproducing an issue where after the Household lookup on a Contact changes,
    // empty Households are deleted, and then a Contact's workflow rule evaluation will fail due to
    // the Household being deleted (despite the fact that it now points to a different Household)
    // in this experiment TestRelatedObject takes the place of Household, as I have installed the NPSP
    // in my dev org, and already had TestRelatedObject from a previous experiment
    
    // follow code from NPSP (taken off Github):
    list<id> blankHHDeletes = new list<id>();
    List<id> oldHouseholds = new list<id>();
    for (Contact c : Trigger.old){                
        if (Trigger.oldMap.get(c.id).TestRelatedObject__c != null){
            oldHouseholds.add(Trigger.oldMap.get(c.id).TestRelatedObject__c);
        }
    }
    Map<id,Integer> oldHouseholdSize = new Map<id, Integer>();
    // get HH contact counts with aggregate query
    AggregateResult[] ar = [select TestRelatedObject__c oldhh, count(id) membercount from Contact where TestRelatedObject__c IN :oldHouseholds group by TestRelatedObject__c];
            
    // create a map from the old household id and its size
    for (AggregateResult a : ar){
         oldHouseholdSize.put((id)a.get('oldhh'), (integer)a.get('membercount'));
    }

    // fill out empty households (they will not be present in aggregate results)
    for (id hhid : oldHouseholds){
         if (!oldHouseholdSize.containskey(hhid)) {
             oldHouseholdSize.put(hhid, 0);               
         }            
    }
    
    // put count=0 households into list for deletion
    List<Id> tbd = new List<Id>();
    for (Contact c : Trigger.old){                
        if ((Trigger.oldMap.get(c.id).TestRelatedObject__c != Trigger.newMap.get(c.id).TestRelatedObject__c) &&
            (oldHouseholdSize.get(Trigger.oldMap.get(c.id).TestRelatedObject__c) == 0)) {
            tbd.add(Trigger.oldMap.get(c.id).TestRelatedObject__c);
        }
    }
    Database.delete(tbd);
}