Set up a new NHibernate console application using our Eg.Core model from Chapter 1. Configure log4net to send the NHibernate.SQL debug output to the .NET trace, just as we did in Chapter 2.

Add the following code to your Main method:

var isbn = "3043";

var query = DetachedCriteria.For<Book>()
  .Add(Restrictions.Eq("ISBN", isbn));

using (var session = sessionFactory.OpenSession())
{
  using (var tx = session.BeginTransaction())
  {
    var book = query.GetExecutableCriteria(session)
      .UniqueResult<Book>();
    tx.Commit();
  }
}

In this recipe, we've used a DetachedCriteria object from the NHibernate.Criterion namespace. This allows us to set up our query without an active session. Later, inside a transaction, we call GetExecutableCriteria to return an ICriteria associated with the session. Finally, we call UniqueResult to return the book.

NHibernate also provides DetachedQuery and DetachedNamedQuery in the NHibernate.Impl namespace for detached HQL queries. The code is given as follows:

var query = new DetachedNamedQuery("GetBookByISBN")
  .SetString("isbn", isbn);

var query = new DetachedQuery(hql)
  .SetString("isbn", isbn);

Detached criteria and queries implement the query objects pattern shown on Martin Fowler's website at http://martinfowler.com/eaaCatalog/queryObject.html.