trigger rollupTrigger on Object__c (after insert, after update, after delete) {
    
    double totalSum = 0;
  
    Object__c myObjects = trigger.new;
    
    if(Trigger.isUpdate || Trigger.isInsert)
    {
            for(Object__c object : myObjects) //assume more than 1 record in trigger.new
            {
              for(ParentObject__c parent : [select Id, Name from ParentObject__c where Id = :object.Parent_Object__c]) //get the parent object
              {
                for(Object__c childObject : [select Id, Name, Number__c where Parent_Object__c = :parent.Id]) //get all the objects that belong to the parent object
                {
                  totalSum += childObject.Number__c;   //sum the field of choice
                }
                parent.Total__c = totalSum;   //set the total in the parent
                update parent                       // update the parent
              }
            }
    }
    else if(Trigger.isDelete)
    {
        Object__c [] oldObjects = Trigger.old;
        
      for(Object__c oldObject :oldObjects)    //assume deletion of more than 1 record
      {
        for (ParentObject__c parentObject : [select id, Name, Number__c, Total__c where Id = :oldObject.Parent_Object__c]) //get the parent object(s)
        {
          for (Object__c childObject : [select id, Name, Number__c from Object__c where Parent_Object__c = :parentObject.id])   //get the records that belong to the parent
          {
             totalSum += childObject.Number__c;  //sum the fields after a record has been deleted
          }
            parentObject.Total__c = totalSum;   //set new total in parent
            update parentObject;                      //update parent
        }   
      } //end oldObject for loop
    } //end else if
    
} //end trigger