updateRelation abstract method
PUT /{resource}/{id}/relations/{relation}/{childId} — updates one child
row, resolved through the relationship (P9): childId is the related
model's own key (the relation's published recordKey), and a child that
is not this parent's answers 404, never a cross-parent write.
Implementation
Future<WriteResult> updateRelation(
String resourceKey,
Object id,
RelationDescriptor relation,
Object childId,
Map<String, dynamic> values,
);