Sum of top 5 elements in NHibernate Linq

Question

I want to receive sum of property in top 5 elements in linq to NHibernate query. If I use below code:

Session.Query<Document>().Take(5).Sum(x => x.Value)

I get 'method not supported' exception.

Any ideas?

NHibernate 3.3.1.4000

Answer 1

You can use this:

Session.Query<Document>().Select(x => x.Value).Take(5).Sum();

NHibernate only supports the Sum overload without an expression parameter.

And for unknown reasons you have to first Select and than Take as you correctly commented.

Answer 2

The problem is that the nhibernate linq provider isn't fully implemented yet. Try to materialise using .ToArray() to memory and then calculate .Sum() using linq-to-memory.

For example:

Session.Query<Document>().Select(x => x.Value).Take(5).ToArray().Sum()

ps: same kind of problems could happens with .Skip() and joins with DefaultIfEmpty()

Answer 3

In NHibernate.4.0.2.4000 the Sum with lambda works also Session.Query<Document>().Take(5).Sum(x => x.Value).

If it is possible to get no rows returned and Value is not a nullable type you need to cast it to a nullable type as the sum will return null when no items are present:

Session.Query<Document>().Take(5).Sum(x => (decimal?)x.Value) ?? 0;