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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);
return primaryParent.SortOrder;
}
Then you can include that in your conventions
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?