delete method
A WriteGone refreshes too: the row is gone either way — the same
outcome as deleting it ourselves, and ResourceViewScreen's record
delete already treats the two as one.
Implementation
Future<WriteResult> delete(Object childId) async {
final result = await _source.deleteRelation(
resourceKey,
id,
relation,
childId,
);
if (result is WriteSuccess || result is WriteGone) await refresh();
return result;
}