Count(Distinct x) using Dynamic Linq Queries

Question

I've been working with the System.Linq.Dynamic extension to see if it will meet the needs of a B.I. application we're working on.  The application needs to dynamically build almost every aspect of the query at run-time.   I've found that dynamic queries will handle almost everything we need.  One issue I'm stuck on is executing the SQL equivalent of select x, count(distinct y) from tbl group by x.

Using the standard linq language, I can perform a distinct count with something that looks like this

 

var qry = from a in db.Orders
group a by new { a.Member.LastName} into g
select new { LastName = g.Key, CountDistinctOrders = g.Select(p => p.Order.P).Distinct().Count() };

This is great, but I'm looking to do this in dynamic linq since I won't know at design time if a Distinct Count will be requested by the user.  Here's where I'm at with the dynamic version

 

var qry = db.Orders.
.GroupBy("new(MemberDemographic.LastName)", "it").
Select("new(Key.LastName as LastName, " +
"Count() as TotalNotDistinct ");

 

Any ideas on how to get Distinct Counts of a column?    Thanks!