Question

How can I do something like this in nHibernate:

select count(*)
from (subquery)

It is a rather simple query in SQL, but the solution is not so obvious in nHibernate. An obvious solution would be something along the line of:

    var rowcount = Session.QueryOver<Entity>()
       .Select(Projections.Alias(Projections.Count(Projections.SubQuery(detachedQuery)), "count"))
        .FutureValue<int>();

However, this results in an ArgumentOutOfRangeException:

Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

This SO answer doesn’t work for me, as I have a more complex grouping.
My question originates from an earlier question where I tried to use ToRowCountQuery, but that function strips groupings form the query.

Answer 1

I found an old post by Ayende which gave me a solution (Counting paged data).

I created my own dialect as described in that post and added the rowcount function to my paged query. And presto, I got my rowcount in a single query to the database.