update method
childId is the child's own key value — the relation's recordKey,
routinely not id; see ResourceDataSource.updateRelation.
Implementation
Future<WriteResult> update(
Object childId,
Map<String, dynamic> values,
) async {
final result = await _source.updateRelation(
resourceKey,
id,
relation,
childId,
values,
);
if (result is WriteSuccess) await refresh();
return result;
}