nHibernate First Impressions

April 25, 2008 at 11:37 am (Object Relational Mapping, Tools) (, , , , , , , )


If I’m going to have to support it, I want to understand it. So, I got going yesterday, installing nHibernate 2.0 and walking through the Quick Start Guide. My C# is a bit rusty, to say the least, but I managed to sqeak by. The Guide is meant for an older version of nHibernate and there have been a few changes made that affect the code displayed. What that means is, I had to do more than simply type up what was presented to me. Which, was actually good because it forced me to do a bit more learning in order to get everything to work.

What I found was interesting. I can see why developers like this. It really does let you treat the database as just another object to program against. More than that, with pretty minimal work (some of which could be eliminated by code generation), you don’t have to think about databases at all. It’s slick, no denying it. More good news, the TSQL queries it generates are very consistent and appropriately formatted parameratized queries. This gives me a great deal of comfort that we’ll get consistent results with this product in place. I also didn’t find it terribly chatty, meaning I didn’t see lots of extraneous calls made to the database. That’s all the good news. Now for the bad. Here’s a sample of the code:

exec sp_executesql N’SELECT user0_.LogonId as LogonId0_0_, user0_.Name as Name0_0_, user0_.Password as Password0_0_, user0_.EmailAddress as EmailAdd4_0_0_, user0_.LastLogon as LastLogon0_0_ FROM users user0_ WHERE user0_.LogonId=@p0′,N’@p0 nvarchar(8)’,@p0=N’joe_cool’ 

Yes, that’s hard to read. Sorry, but that’s how it comes out, so that’s part of what you’ll be living with. First off, and this is usually a minor nit, but can be a serious problem, the generated code from the quick start guide did not provide a schema for the table, ‘users’. In tiny little procs like this, that’s neither a performance hit nor is it likely to cause expensive recompiles. If the tool generates larger more complex queries, that is potentially an issue. Second, and much more important, here’s the DDL to create the ‘users’ table.

CREATE TABLE [dbo].[users](
        [LogonID] [nvarchar](20) NOT NULL DEFAULT (‘0’),
        [Name] [nvarchar](40) NULL DEFAULT (NULL),
        [Password] [nvarchar](20) NULL DEFAULT (NULL),
        [EmailAddress] [nvarchar](40) NULL DEFAULT (NULL),
        [LastLogon] [datetime] NULL DEFAULT (NULL),
PRIMARY KEY CLUSTERED
(
        [LogonID] ASC
) ON [PRIMARY]
) ON [PRIMARY]

Nothing fancy. But can you spot the discrepancy? The clustered primary key is on a NVARCHAR(20) column. The code that calls it is defaulting each parameter, not to the size of the column, but to the amount of data supplied, NVARCHAR( 8 ) in this case. That very simple query could use implicit data conversions and cause the indexes to not be used. That isn’t a problem here, but it’s something I’m going to be keeping a close eye on.

Ultimately, it looks like this is something I can support. I’m going to spend more time with it to see if I can work out these minor kinks and to introduce joins and more complex data structures to see how well it deals with things. This should make things interesting for a while. I still wonder how we’re going to deal with this in production.

1 Comment

  1. nHibernate, First Look at TSQL « Home of the Scary DBA said,

    […] because I’m seeing lots of implicit data conversions on the parameters being passed in, as I noted a couple of years ago. At this point in time, I would say, at best, I’m very underwhelmed by what nHibernate is […]

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