Good thing is that, in case of an inefficient query, NHibernate puts a warning using log4net:

log.Warn( "firstResult/maxResults specified with collection fetch; applying in memory!" );

I wanted something like this:

[Test]
public void FindUsersBy_QueryWithLimit_LimitsOnSQLSide()
{
    using (LogChecker logChecker
        = new LogChecker("NHibernate", Level.Warn))
    {
        // Execute query using NHibernate

        // ....

        Assert.IsEmpty(logChecker.Messages);
    }
}

The problem is that it’s not that easy to attach MemoryAppender for a duration of unit test to the specified logger.

Anyway here’s the code:

public class LogChecker : IDisposable
{
    readonly Logger _logger;
    readonly Level _previousLevel;
    readonly MemoryAppender _appender = new MemoryAppender();

    public LogChecker(string logName, Level levelToCheck)
    {
        _logger = (Logger)LogManager.GetLogger(logName).Logger;
        _logger.AddAppender(_appender);
        _previousLevel = _logger.Level;
        _logger.Level = levelToCheck;
    }

    public List<string> Messages
    {
        get
        {
            return new List<LoggingEvent>(_appender.GetEvents())
                  .ConvertAll(x => x.RenderedMessage);
        }
    }

    public void Dispose()
    {
        _logger.Level = _previousLevel;
        _logger.RemoveAppender(_appender);
    }
};