public function changed($attrs=false) 
    {
        $changes = array();
        $cls     = get_called_class();

        foreach (self::$_columns[$cls] as $column) {
            // if is the primary key or one of the automatic columns, skip
            if ($cls::getPK() == $column || in_array(strtolower($column), array("created_at","updated_at"))) {
                continue;
            }

            $cur = $this->get($column);         // current value
            $old = $this->get($column, false);  // old value

            // if same value, didn't change
            if ($cur == $old) {
                continue;
            }

            // set the value as the changed column name
            $value = $column;

            // if checking the attributes, return an array with both values
            if ($attrs) {
                $value = array($old, $cur);
                $changes[$column] = $value;
            } else {
                array_push($changes, $value);
            }
        }
        return $changes;
    }