For someone who got stuck with NHibernate 1.1 it was actually surprisingly hard to find much about projections. Before you had to do different queries for a COUNT or a SELECT and you couldn’t use ICriteria.
int count = (int) session.CreateQuery(string.Format(
"SELECT COUNT(instance) FROM Login instance WHERE Role='{0}'",
TransitLoginRole.Administrator.ToString()))
.UniqueResult();
This is resolved with a projection.
int count = session.CreateCriteria(typeof(Login))
.Add(Expression.Eq("Role", TransitLoginRole.Administrator.ToString()))
.SetProjection(Projections.Count("Id"))
.UniqueResult<int>();
IList<Login> logins = session.CreateCriteria(typeof(Login))
.Add(Expression.Eq("Role", TransitLoginRole.Administrator.ToString()))
.List<Login>();
Nice and easy.