Well, it seems that I managed to lower the timeSpan.TotalDays (C# geek joke) between posts from 150 to 130 or so…

During all these 130 days, I have been VERY busy using and learning NHibernate, and then using and learning NHibernate again. I mean, it is not an easy thing if you do not have a solid background of what this is all about.

Ok, if you try some simple things, then no problem at all, not very background needed. Just create an object, open a NHibernate session, call save(myObject) on that session and you are done.

But… what happens when you have a complex object tree (the usual in almost every not-that-simple application) with collections of objects that have also collections, etc. ? Well, you probably will end up with a REALLY BIG SQL query sent to the server, with a lot of INNER and OUTER JOINs just to only retrieve a list of some objects.

NHibernate newbie (NHn): What? Is not all this ORM thing (NHibernate et al.) be here to help reduce the amount of time spent in developing a RDBMS application?

Opera362: Yes! but you first have to master the tools you are using, nobody knows what bad use you can make of them if you do not know how are they working.

NHn: Then, why is NHibernate doing such those REALLY BIG SQL queries just to obtain some small list of objects?

Opera362: Well, first of all, because we probably unintentionally asked it to do just that… How can it be? If you start with NHibernate, it is very easy to ask for the WHOLE object tree if you do not explicitly instruct it to not do it.

NHn: But… I have read about some “lazy initialization”, I think this is what you need here!! I think that this is enabled by default with NHibernate >= 1.2.0

Opera362: Yes, you are right, but “use with caution”, because this drives you to the BIG problem with ORM systems: The “N+1 Select problem”. Ayende (one of NHibernate contributors) describes it and tells how to combat the problem here.

NHn: Well, so I understand now that I must have that in mind when writing the queries. SetFetchMode( ) will be my best friend from now.

Opera362: SetFetchMode? I think you are talking about the ICriteria API. What about the IQuery one? do you know of its existence?

NHn: Well, I know it exists (with that HQL thing and all), but right now I am using only the ICriteria API, which I feel it is more object oriented and, all in all, an easier thing to learn.

Opera362: Good. I also think ICriteria is more “object oriented” (maybe I must say “intellisense” oriented?) but please have a look at HQL and learn it. It is the MOST powerful query API NHibernate has.

You can do so many things with it, that when learnt, you will find yourself using ICriteria only with normal queries (the word “normal” here does not mean easy or simple). When you must do some Report Queries, or use some aggregation functions, even import some mixed class fields into a new (transient) class to flat an object tree, you need to know (if not master a bit) HQL and the IQuery API.

NHn: But, I think HQL is not easy to learn! Are you really sure I will need it?

Opera362: Absolutely. And after all, HQL is not that difficult it can seem at the beginning. I think the “HQL fear” can be only understood when you are first learning NHibernate by itself, the ORM concept and whatever is involved. I mean, if you begin with NHibernate, then I recommend to use only the ICriteria API, because I think it is easier. Then, when you have a more solid knowledge of NHibernate, the jump to HQL will be ridiculously easy. You will see. Trust me. And then you will be REALLY happy, because HQL will help you to solve those slooooow queries and bring a smiley to your face… again!

NHn: Well, I have been working with NHibernate for some weeks, so I think I must start to learn HQL right now. Any suggestions from where should I start?

Opera362: Well, maybe “some weeks” is not time enough to be faced with the previous problems I told you, but maybe you are starting to surface those.

In the past, there was only a book you could use as a reference (Hibernate in action, and later Java Persistence with Hibernate but for NHibernate users the former is the better) but now I firmly recommend the excellent book NHibernate in Action (by Kuate, aka KPixel in the Nhibernate forum)

NHn: But wait, this book is not print yet!!

Opera362: Yes, but you can buy the book using an “Early access program” that the publisher has with this and some others books. They now email you a PDF (after paying, of course) with the chapters already sent by the author, and later when the book it is finished they will send you the final PDF (or the final print book) when it is ready. In this case, the book it is almost complete (12 chapters out of 12 are in, and only 4 appendices are left).

Kuate must be relaxing (friendly pun intended) after a hard work, and he can not find time to finish the remaining four appendices, but sure he will (maybe he is even waiting for NHibernate 2.0!! I think not a good idea…). And after reading the whole book, I guess all 4 they will be brilliant.

I can not recommend this book highly enough. It is very good and a very valuable help with NHibernate. You will understand me when you read it and see your problems depicted and solved.

NHn: Ok, anything else you can tell me?

Opera362: Sure I would tell you something more, but remember that I am only a “not-so-newbie” on NHibernate. I just started 130 days ago. But I have already been caught inside some REALLY BIG SQL Queries, using 100% CPU on a SQL Server 2005 Express, and wondering WTF NHibernate was doing with it.

So, one thing you must also master is log4net, the open source logging library NHibernate uses to log its messages (do not worry, maybe it seems hard at the beginning, but it is very easy to use when you are able to find how to configure it).

Another thing you must know it is the NHibernate parameter “show_sql” that will be also a very good friend of you when debugging those REALLY BIG SQL queries.

All in all, with show_sql=true, log4net, and HQL, your REALLY BIG (and slow) SQL Queries will be part of the past. And you could obtain a list of objects in milliseconds, not on “million of seconds”.

NHn: I can not wait more… I want to test whatever you told me. See you!! bye!!

Opera362: Oops… you left without me warning you about some “advanced” technique that I now just do not recommend for beginners, called “One-to-one mapping”. It is a double-edged sword, with a lot of power if well used, but dangerous enough if misused to do not recommend at all to newbies. But I will discuss this in a future post, in the meantime, my advice is to use two uni-directional many-to-one associations between two classes that have a logical “1-to-1″ relationship. Yes, I know that probably Sergey Koshcheyev (one of NHibernate main developers) would not agree on this, but just as you and I are not (yet?) NHibernate masters, I do not mind using a NHibernate < 1.0 thing until I can fully understand the pros and cons of more advanced techniques.

This is a summary of these 130 days I have spent with this nice piece of software called NHibernate. I learnt how to love it. It has not to be blind love, like you may feel at first, but rather a intelligent one (knowing its pitfalls and how to avoid them).

See you later… I hope that in timeSpan.totalDays < 130 !!!

One of the BEST articles, if not the best, I have read is NHibernate Best Practices with ASP.NET by Billy McCafferty. It’s brilliant! But if you want to learn NHibernate from zero, that reading it’s not for you… yet.

You previously need a background in doing some simple things with NHibernate. The NHibernate Quick Start Guide is a good starting point. You will need to install SQL Server 2005 (the express edition it’s free and well-featured) to follow it, but NHibernate it’s not limited only to that database.

So that’s all for now. I’m going to try to get my own program with my own classes, my own collections and so on work with NHibernate + DAO objects (read NHibernate Best Practices with ASP.NET when you are ready, and enjoy!).

Maybe nobody is going to read this lines, but I want to write them in order to return to the Internet at least a little bit of the much valuable information I’ve gathered from her (well, Internet has to be female, hasn’t she?).

Anyway, I want to try this Web 2.0 thing by myself. So let’s start!

I will test if it fit my needs (obviously, YES)

Best regards