NHibernate QueryOver select entity and aggregates

Question

What I want to do is display a simple grid of data which contains the entity data, and the aggregate data of its children. For example lets use a Order and line items. I want to display the order information, and the count of line items.

OrderID, OrderDate, NumOfLineItems

Now normally in SQL you can do it many ways. But this is the only way I could think of that might work when translating to NHibernate.

SELECT o.OrderID, OrderDate, NumOfLineItems
FROM #Orders o
INNER JOIN
(SELECT o2.OrderID, COUNT(*) As NumOfLineItems FROM #LineItems l
INNER JOIN #Orders o2 ON o2.OrderID = l.OrderID
WHERE UserID = 1 GROUP BY o2.OrderID) t1 ON o.OrderID = t1.OrderID
WHERE UserID = 1

I know there are other ways, but I'm trying to think of ways that NHibernate will allow me to do using QueryOver syntax. I do not want to use derived columns. I am trying to avoid writing SQL.

For my entities I have an Order entity, and an AggregatedOrder entity, which will be my DTO in this case, and I plan to use the transformer aliastobean to copy the data into it.

I just have absolutely no idea how to figure this out.

All I have so far:

        QueryOver<LineItem> x = QueryOver.Of<LineItem>()
            .SelectList(p => p .SelectCount(l => l.Id).WithAlias(() => itemAlias.NumOfLineItems))
            .JoinQueryOver<Order>(l => l.Order)
            .Where(o => o.UserID == userID)


        var y = session.QueryOver<Listing>()
            .JoinQueryOver<Bid>(x); // no idea whats going on here

Answer 1

Given:

public class Order
{
    public virtual int OrderId {get; set;}
    public virtual DateTime OrderDate {get; set;}
    public virtual IList<LineItem> LineItems {get; set;}
}
public class LineItem
{
    public virtual int Id {get; set;}
    public virtual string Description {get; set;}
}

To query a projection of Order + Aggregated LineItem using QueryOver API you can do the following:

OrderDto orderDto = null;
LineItem items = null;
var results = session.QueryOver<Order>()
     .JoinAlias(o => o.LineItems, () => items)
     .Select(Projections.ProjectionList()
      .Add(Projections.Property<Order>(o=>o.Id).WithAlias(()=>orderDto.OrderId))
      .Add(Projections.Property<Order>(o=>o.DateOrdered).WithAlias(()=>orderDto.DateOrdered))
      .Add(Projections.Count(()=> items.Id).WithAlias(()=>orderDto.ItemCount))
     )
     .TransformUsing(Transformers.AliasToBean<OrderDto>())
    .List<OrderDto>();