Reading and writing a Reference<T>

var ev = db.Get<IEvent>(eventId);

ev.Cover.IsSet();                       // is a target set?
ev.Cover.Id;                            // the Guid, Guid.Empty when unset

if (ev.Cover.TryGet(out var asset))     // loads the target
    Console.WriteLine(asset.FileName);

var img = ev.Cover.Get();               // throws when unset

ev.Cover.Set(someAssetId);              // by id
ev.Cover.Set(someAsset, db);            // by node
ev.Cover.Clear();
db.Update(ev);                          // a reference is node data: save it with Update