Using NHibernate Projections.Max in a Subquery

I found myself in the need of doing a Subquery in NHibernate recently that would fetch the max value of a column as the data I was working with was versioned.

The SQL that I needed to replicate was essentially the follow:

SELECT * 
FROM databaseTable 
WHERE property = 'value' 
     AND version = (SELECT MAX(version) FROM databaseTable WHERE property = 'value')

After a bit of investigation I found the following solution:

using(var session = _database.OpenSession())
{
     var detachedCriteria = DetachedCriteria.For(typeof(DatabaseRecord))
          .Add(Restrictions.Eq("Property", value))
          .SetProjection(Projections.Max("Version"));

     var record = session.CreateCriteria(typeof (DatabaseRecord))
          .Add(Restrictions.Eq("Property", value))
          .Add(Subqueries.PropertyEq("Version", detachedCriteria))
          .UniqueResult<DatabaseRecord>();

     if (record == null)
          throw new Exception("No records were found where Property = " + value);

     return record;
}

100% CPU Usage

I recently found myself trying to solve an issue that we had at work, whereby one of the load balanced servers would shoot up to 100% CPU usage, and then remain there until we recycled the application pool of the site that was causing the issue.

After some investigation a colleague of mine said, "Hold on, isn't this lock object supposed to static?" Sure enough we had a block of code similar the following:

public class SomeClass
{
     private readonly IDictionary<string, string> _someList;
     private readonly object _lockObject = new object();

     public SomeClass()
     {
          _someList = new Dictionary<string, string>();
     }

     public string Get(string key)
     {
          string value;
          if(!_someList.TryGetValue(key, out value))
          {
               lock (_lockObject)
               {
                    if (!_someList.TryGetValue(key, out value))
                    {
                         value = GetValueFor(key);
                         _someList.Add(key, value);
                    }
               }
          }

          return value;
     }
}

We fixed the code by simply making '_lockObject' static as follows:

private static readonly object _lockObject = new object();

When we pushed this to live and all was fixed. It was a pretty obvious mistake but the symptoms were not immediately obvious to us as to what to look for so i thought I would put this out there on the tinterwebs for if anyone else encounters the same issue.