Fluent NHibernate: cannot convert lambda expression error

If you received this kind of error...

Cannot convert 'lambda expression' to non-delegate type Options

...just add System.Core in your References

Performing ORDER BY on DISTINCT on Linq to NHibernate(version 3)

You will received errors when doing OrderBy on Distinct result on Linq to NHibernate (NHibernate 3) at the time of this writing:

var cat = session.Query<Product>().Select(x => x.Category).Distinct().OrderBy(s => s);

Convert it to:

var cat = session.Query<Product>().Select(x => x.Category).OrderBy(s => s).Distinct();

Alternatively you can do this, which is quite neat:

var cat = 
        (from c in session.Query<Product>()
        orderby c.Category 
        select c.Category).Distinct();

Note the last two codes produces this(which is performant, heaven thanks):

select distinct category 
 from product 
 order by category asc

Not this:

select distinct category
 from 
 (select category from product
 order by category)

Be aware that if you are using Linq to SQL, the 3rd code construct cannot construct proper query (ORDER BY is omitted on generated query, silent error). Documented here: http://programminglinq.com/blogs/marcorusso/archive/2008/07/20/use-of-distinct-and-orderby-in-linq.aspx

He advises to move the orderby out of query to .Distinct() extension method.

var cat = (from c in session.Query<Product>()    
    select c.Category).Distinct().OrderBy(s => s);

Which leads to attaching two extension methods on the query just to make Linq to SQL emit the correct SQL. Which IMHO, renders the whole point of making Linq as query-like as possible lame.


I prefer the Linq to NHibernate approach than Linq to SQL. Not because NHibernate is database-agnostic(but it certainly adds appeal), but for the reason that it correctly informs the programmer that it cannot do something by not performing silent errors; it fail fast.

Here's the error emitted when performing OrderBy on Distinct expression:

Unhandled Exception: System.NotSupportedException: Operation is not supported.

Unix transport error when unit testing NHibernate for Postgresql under Mono

If you happen to encounter the following error while doing unit testing for NHibernate for Postgresql under Mono...

Internal error
        RemotingException: Unix transport error.

...Change your Npgsql version to a Mono one, then that error won't happen.

That error will appear on unit testing if you are using MS .NET version of Npgsql (example: http://pgfoundry.org/frs/download.php/2868/Npgsql2.0.11-bin-ms.net4.0.zip) under Mono. When unit testing under Mono, you must use Mono version of Npgsql (example: http://pgfoundry.org/frs/download.php/2860/Npgsql2.0.11-bin-mono2.0.zip)

Weird problem, the error only appear when the code is run under Unit Testing(built-in in MonoDevelop). But when run independently, MS .NET version of Npgsql will run just fine under Mono. Anyway, to make matters simple, use Mono version of the component if you are building Mono stuff

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