Using LINQ-to-NHibernate Sort an IQueryable of T based on a property value within T's Children

Question

Though the project in which I'm having this issue addresses a different domain, I've found a metaphor that may help to present my question clearly:

Let's say that you had to develop a Forum web site, and it was a requirement that the default view for the site needs to display Forum Threads ordered by the threads with the most recent Forum Posts. How could this be accomplished using Linq-to-Nhibernate?

Here is an example of how those Entities might look:

namespace MvcSandbox.Models
{

public class ForumThread : Entity
{
    public virtual string Topic { get; set; }
    public virtual IList<ThreadPost> Posts { get; set; }
}

public class ThreadPost : Entity
{
    private ForumThread _thread;

    public virtual ForumThread Thread
    {
        get { return _thread; }
    }

    public virtual DateTime PostDate { get; set; }

    public virtual string PostText { get; set; }
}
}

So, given an IQuerable of ForumThread, how could Linq be used to append an OrdbyBy clause that will sort the ForumThreads based on the Max(PostDate) of their respective Posts collection? And for performance reasons, I'm trying to find a solution that will enable the Order By to occur in the database rather than in memory.

Any input would be greatly appreciated.