How to have NHibernate use a SQL MERGE INTO statement

On our project we were noticing a massive performance penalty when sending a number of records to the database to either be inserted or updated via the standard NHibernate framework syntax, so we decided to see if we could do anything about this. Turns out that we were able to alter the query that NHibernate normally generated, and change it into a MERGE INTO type statement, which meant a huge