System.InvalidCastException: Unable to cast object of type ‘Oracle.DataAccess.Client.OracleConnection’ to type ‘System.Data.Common.DbConnection’

This was one of those awesome “doesn’t happen on any of my machine” errors.  After some mucking around we determined that the client machine was using the Oracle 10g  R1 client.  The machines and VMs here we used for testing were all running either 11g, or 10g R2.  Doh!

The simple resolution to this was to modify the NHhibernate config and add the following property:

   1: <property name="hbm2ddl.keywords">none</property>

Depending on your underlying databases and mappings this could cause problems with your code project.  Hopefully you’re not using reserved words, or bad Oracle syntax =).

Under the hood, the problem seems to be that the Oracle10gDialect does not provide an implementation of IDataBaseSchema for 10gR1.   Fabio Maulo has provided some sample code and the steps on how to create the appropriate metadata for your database and submit it to the NHibernate project for inclusion.

NH used to assume Castle – but now it needs to be configured.  In addition to the Castly proxy, there is a new LinFu proxy available too.   Don’t bother with NHibernate the release notes! , Davy Brion has it all for you here =). 

I opted to go with the Castly Dynamic Proxy – I’m not in a position to do any performance tests between– Castle has been around a long time and has a pretty big user base.  Good post on stackoverflow on this subject.  However, LinFu would have won if I was basing my choice on name alone…

After a bit of fiddling, I’m back up and running with NH 2.1 and shiny new validation attributes.   

mmmm validation…

NHibernate was a tough curve for me.  Initially, I started testing code generators.  After spending a significant amount of time reading, testing, and pulling out hair I came to the conclusion that NHibernate code generation is still not there.  From my experiences Genwise is probably the best code generator out there right now – but its still lacking in some areas.  More importantly however, I learned that code generation is not a good place to start when implementing NHibernate for the first time.  There is so much going on under the hood – in my opinion you need to start off doing a reasonable size project by hand.  Code generation has significant benefits, because the generated code is so similar with simple replacements it just makes a lot of sense once you wrap your head around what’s going on with NHibernate.

As per my previous post on ReSharper when implementing nHibernate by hand, ReSharper is a must have.  Just the other day I found a *great* ReSharper NHibernate Plugin.  When editing NHibernate HBM XML files, it does a bunch of validation right in the editor.  Saves you a lot of time finding problems without having to actually run a unit test and debug sometimes cryptic error messages.

I bought a copy of NHibernate In Action.  I would recommend this book to you only if you have no experience, or understanding of Object Relational Mappers.  As the book stands right now, it’s not really a good developer reference and only covers really basic examples.  The documents that helped me the most to get everything up and running was The NHibernate FAQ.  The NHibernate documentation is pretty cryptic.  Don’t read this stuff 12 hours into your day.  No amount of RedBull will give you the focus needed to make sense of it.  In the first 8 hours of the day, its quite helpful.

I primarily work in VB.NET.  For this project however, I switched to C#.  Maybe now, the my colleagues at Bear Mountain will stop giving me a hard time =).  Seriously though.  Examples are a LOT easier to come by in C#, probably 10:1.

Finally, get NUnit running.  I had a lot of problems using the built in Microsoft Test projects with NHibernate – so I’ll be sticking with NUnit. If you’ve never done unit testing (you know who you are) do it.  Particularly in this case – its crucial.  You (Again you know who you are =]) used to write simple winform apps, and put a bunch of buttons on them.  Don’t do that. 

So, if you are one of the RADE non-developer users and you read on this far.  Congrats.  This milestone will not directly help you with your use of RADE.  However, it’s a large step forward for the underlying architecture – and you will benefit from that. 

Well back to Visual Studio I go..