The case is:
Search for products and order by its SortOrder property (from NodeEntryRelation).
How to index SortOrder property along with CatalogId, CatalogNodeId and include it in search results?
You can include almost anything with your index using extension method, something like this (pseudo code)
public static in SortOrder (this EntryContentBase entry)
var nodeEntryRelations = _relationRepository.GetParents<NodeEntryRelation>(entry.ContentLink);
var primaryParent = nodeEntryRelations.FirstOrDefault(x=>x.IsPrimary);
Then you can include that in your conventions
Fantastic, Quan Mai, thank you very much!