string res = string.Empty;
 
try
{
    IQuery query = session.CreateQuery("select ao.CostCenterAccount from " +
                                       "AssetObservation as ao where ao.Asset.ID=:KEY " +
                                       "and ao.Date<:DATETIME order by Date asc");
    query.SetInt64("KEY", asset.Id);
    query.SetDateTime("DATETIME", dateTime);