We have a productmodel with a list containing the episerver category ids.
Now we want to order products by the selected category. We do not want to filter out products that do not have the categories, just put the ones with the category at the top.

var storeCategory = GetCategory(sortBy);
productCollection.Products = productCollection.Products
.OrderByDescending(x => x.Categories.Contains(storeCategory.ID) ? 1 : 0)
.ThenBy(x => x.SortIndex).ToList();

What we want is

var searchResult = query.TermsFacetFor(x => x.FeatureCategories, request => request.Size = 1000)
.TermsFacetFor(x => x.FeatureList, request => request.Size = 1000)
.TermsFacetFor(x => x.Sports, request => request.Size = 1000)
.OrderByDescending(x => x.Categories.Contains(storeCategory.ID) ? 1 : 0)
.ThenBy(x => x.SortIndex).ToList();
.Skip(page * pageSize)
.Take(pageSize)
.GetResult();

That's the code for the orderby but this is not done on the actual query, so some products are lost.
We tested using Match(storeCategory.ID) but that is not allowed within a OrderBy, only Filter from what I understood.

BR,

Henrik