How to configure Fluent NHibernate to output queries to Trace or Debug instead of Console?

I can see from forum and blog posts everywhere that lots of others before me have looked for a way to get the SQL statements as they're being prepared for execution. The answer typically is something along the lines of "you can't", or "you shouldn't".

Whether I should or not, that's what I wanted.

After hours of searching, investigation and failed attempts, and finally I came up with this.

Write up an interceptor:

using NHibernate;
using System.Diagnostics;

public class SqlStatementInterceptor : EmptyInterceptor
{
    public override NHibernate.SqlCommand.SqlString OnPrepareStatement(NHibernate.SqlCommand.SqlString sql)
    {
        Trace.WriteLine(sql.ToString());
        return sql;
    }
}

Of course, you don't have to Trace.WriteLine() here, you could write it to a log file, or whatever else you need.

In your connection manager, hook up your Interceptor like so:

protected virtual void Configure(FluentConfiguration config)
{
    config.ExposeConfiguration(x =>
                                   {
                                       x.SetInterceptor(new SqlStatementInterceptor());
                                   });
}

It's not that complicated. From my perspective, certainly easier than trying to get all this XML pushed through Fluent to NHibernate - since Fluent abstracts the XML file away.

Keep in mind, you can only have a single Interceptor - so you may need to integrate this feature with your existing Interceptor, if you already have one. On that note, you might want to give it a broader name - e.g. MyAppInterceptor, so as not to imply a specific purpose, because you may want to add other features to it later.

You probably want to use log4net, not ShowSql. Here is some configuration to send queries to Debug:

  <configSections>
    <section name="log4net"
     type="log4net.Config.Log4NetConfigurationSectionHandler,log4net" />
  </configSections>

  <log4net debug="false">
    <appender name="WindowsDebugOutput" type="log4net.Appender.DebugAppender,
         log4net">
      <layout type="log4net.Layout.PatternLayout,log4net">
        <param name="ConversionPattern"
              value="%d{ABSOLUTE} %-5p %c{1}:%L - %m%n" />
      </layout>
    </appender>

    <logger name="NHibernate.SQL" additivity="false">
      <level value="DEBUG" />
      <appender-ref ref="WindowsDebugOutput" />
    </logger>
  </log4net>

And then call this from your code before opening an NHibernate session:

log4net.Config.XmlConfigurator.Configure();

When you add a reference to the log4net DLL, make sure to set its "Copy Local" property to "true".

This isn't specific to FluentNHibernate, it works the same in any variant of NHibernate.

I have not tried this with SQL Server, but with SQLite, the following code will show generated SQL in the Output window (Debug menu -> Windows -> Output, in VS2008).

The "Show output from:" combo box in the Output window should be set to "Debug" - VS2008 did that for me automatically.

            sessionFactory = Fluently.Configure()
                .Database(SQLiteConfiguration.Standard
                            .UsingFile(DbFile)
                            // Display generated SQL in Output window
                            .ShowSql()
                          )
                .Mappings(m => m.AutoMappings.Add( GetAutoPersistenceModel() ))
                .BuildSessionFactory()
                ;

A word of warning - turning this on can slow down execution considerably.