Castle ActiveRecord .net C# how to implement IndexOf

Question

I would like to implement IndexOf for ActiveRecord mapped class by using ordering and custom filtering criteria something like this:

long MyEtityType.IndexOf(
    MyEntityType myEntityInstance,
    DetachedCriteria filter,
    String propertyNameOrderBy,
    Boolean isAscending
)

What is the best way to do this ? Can I use linq something like this:

MyEntityType.Querable.Where.IndexOf

Does this provide correct SQL or will it fetch all items?