c# - NHibernate / LINQ select MAX in subquery?

Question

I've been trying to get the following SQL

SELECT * 
    FROM dbo.VirtualMachines vm
    WHERE vm.SequenceId IN (
        SELECT MAX(SequenceId) FROM dbo.VirtualMachines GROUP BY RequestId
    )
    AND vm.DeletedBy IS NULL

...into a LINQ-query for use with NHibernate.

I've been able to get a variation on this working which is based on a corrolated sub-query:

var allvms = from vms in this.currentSession.Query<Entities.VirtualMachine>()
             where vms.DeletedBy == null
             where vms.Id == (
                 from activeVms in this.currentSession.Query<Entities.VirtualMachine>()
                 where activeVms.RequestId == vms.RequestId
                 orderby activeVms.Id descending
                 select activeVms.Id).First()
             orderby vms.RequestId
             select vms;

...which gives me...

SELECT *
    FROM dbo.VirtualMachines vm
    WHERE vm.SequenceId IN (
        SELECT TOP 1 zvm.SequenceId From dbo.VirtualMachines zvm WHERE zvm.RequestId = vm.RequestId ORDER BY zvm.SequenceId DESC
    )
    AND vm.DeletedBy IS NULL

...however I'd rather use the MAX() version as (having profiled SQL Server) it's a more efficient query for the dataset I'm working with. Unfortunatly, I can't work out how to wrangle LINQ to give me the query.

I know I can do:

from vms in this.currentSession.Query<Entities.VirtualMachine>()
    group vms by vms.RequestId into vmReqs
    select new { 
        LatestSeqId = vmReqs.Max(vms => vms.SequenceId) 
    }

which gives me the sub-select (SELECT MAX(SequenceId) [...]), but I can't see how to combine this with the query I've already got to do an IN. It's possible I'm approaching this in too-SQL a mindset, and I'm trying to approach the query as I would in SQL and there's some other technique I've missed.

Answer 1

Something like this:

var subQuer = from vms in this.currentSession.Query<Entities.VirtualMachine>()
              group vms by vms.RequestId into vmReqs
              select vmReqs.Max(vms => vms.SequenceId);

var outerQuery = from vm in this.currentSession.Query<Entities.VirtualMachine>()
                 where subQuery.Contains(vm.SequenceId) && vm.DeletedBy == null;