loadBelongsTo method
Load the related entity for a belongsTo relation.
owner is the owner entity; foreignKey (set in belongsTo) is the field on owner holding the related id.
Implementation
Future<R?> loadBelongsTo(T owner) {
final fkValue = _getFieldValue(ownerCollection, owner, foreignKey);
if (fkValue == null) return Future.value(null);
final id = fkValue is int ? fkValue : int.tryParse(fkValue.toString());
if (id == null) return Future.value(null);
return relatedCollection.getById(id);
}