reference with C<filter_from_storage> and C<filter_to_storage> set to either
a method name or a code reference. In either case the filter is invoked as:
- $row_obj->$filter_specification ($value_to_filter)
+ $result->$filter_specification ($value_to_filter)
with C<$filter_specification> being chosen depending on whether the
C<$value_to_filter> is being retrieved from or written to permanent