Question

Is it possible to extend NHibernate to add support for aggregate functions that is unique to a database system? I have a handful of queries that use the array_agg() function (and others) in PostgreSQL that I’d like to convert to HQL/ICriteria.

Thanks!

Answer 1

Yes. It’s possible.

Take a look:
http://ayende.com/Blog/archive/2006/10/01/UsingSQLFunctionsInNHibernate.aspx

You could do something like this:

public class MyDialect : PostgreSQLDialect
{
       public MyDialect()
       {
              RegisterFunction("dbo.myfunction", new
              StandardSQLFunction(NHibernateUtil.String));
       }
}

Then you can use myfunction in your HQL statements. You just need to register the Dialect in your NH Configuration.

Edit:

Good news, this should work for Criteria queries as well.

This is code (from Reflector) in PostgreSQLDialect

base.RegisterFunction("iif", 
    new SQLFunctionTemplate(null, "case when ?1 then ?2 else ?3 end"));

So in you ICriteria, you could do:

Projections.SqlFunction("iif", NHibernateUtil.Boolean, foo, bar, baz...

All this should work because you’re extending a NH Dialect.

HTH