orderByFields method
Use one field per call; chained calls preserve explicit sort precedence.
Implementation
TableQuery<R> orderByFields(Map<String, Direction> fields) {
if (fields.length != 1) throw ArgumentError('Order one field per call');
final field = fields.keys.single;
definition.column(field);
return _copy(
orders: List.unmodifiable([..._orders, (field, fields[field]!)]),
);
}