Max in linq to NHibernate for not exist data in database

Question

I have a query by linq to NHibernate.

var q = SessionInstance.Query<Request>().Max(e => e.Code);

If Request table has no rows, execution of this query raises GenericADOException with this message :

{"Could not execute query[SQL: SQL not available]"}

{"Value cannot be null.\r\nParameter name: item"}

What should I do?

score 13 · Accepted Answer · answered Jun 12 '12 at 14:00

13

Try this

SessionInstance.Query<Request>().Max(x => (int?)x.Code);

answered Jun 12 '12 at 14:00

Marian Ban

Why does the cast make this work? Was `x.Code` something else than `int?`? (I just want to learn, not promoting my own answer). – Gert Arnold Jun 12 '12 at 14:31
@GertArnold I don't know, it just works:) But i think that it is something related to Nhibernate linq provider. – Marian Ban Jun 12 '12 at 14:37
1

Ok, thanks for your response. Maybe someone can shed some light on this. – Gert Arnold Jun 12 '12 at 14:40
1

Your Max function might return null if for example there is no rows in the database table, therefore you casting it properly `(int?)x.Code`. There's gonna be no difference in generated SQL. – Alex M Nov 28 '12 at 15:51

score 1 · Answer 2 · answered Jun 12 '12 at 14:18

1

I think this should work with Linq-to-Nhibernate:

var q = SessionInstance.Query<Request>().Select(e => e.Code)
    .DefaultIfEmpty().Max();

or maybe DefaultIfEmpty(<some value>).

answered Jun 12 '12 at 14:18

Gert Arnold

Your solution for many records in database for performance is not suitable. – Ehsan Jun 12 '12 at 14:22
Why not? DefaultIfEmpty will not fetch more records, it will only add a value if the sequence is empty. – Gert Arnold Jun 12 '12 at 14:25
3

DefaultIfEmpty for NHibernate linq provider is not implemented and throws exception as of NHibernate 3.3.1.4000 version – Darius Kucinskas Jul 05 '12 at 06:58
3

Indeed - "The DefaultIfEmptyResultOperator result operator is not current supported" – PandaWood Feb 18 '14 at 04:59

2 Answers2