NHibernate Mapping Examples
Posted by Davy Brion on July 15th, 2007
When you’re starting out with NHibernate, it’s sometimes hard to find good examples. Most examples online are too simple, or are incomplete (just the mappings, but no code for instance). It’s always easier if you have some examples that are large enough, but still small enough to be easy to grasp. So i created mappings and classes for the Northwind tables. I figured this could be useful reference material for anyone new to NHibernate so i’m making the whole thing available for everyone. You’ll find examples of one-to-many, many-to-one and many-to-many associations in there. I used a couple of different cascade options for the associations, depending on the constraints on the tables. I’ve also included about 60 unit tests to verify the mappings are working correctly. These tests should also give you a good idea about how NHibernate deals with certain mappings and options.
I did modify the Northwind database here and there… the zip file contains all the sql create scripts. This is what my version looks like:
And the class diagram looks like this:
I do want to make it clear that this is just an example. I’m not saying that this how you should map your objects and their associations to tables and their relationships. I pretty much provided every possible association in this example, whereas on a real project i’d only create the associations that i actaully need to implement the required functionality. But for the purpose of this example, i thought it would be a good idea to provide as much as possible.
You can download it here
July 16th, 2007 at 12:00 am
Really nice, I just took a look through the mappings in the solution. I just wondered if you could have supplied some examples of mappings using lists and maps as well. One can hardly ever find examples using these types of mappings, while bags and set are often demonstrated.
Keep up the good work,
Julian.
July 16th, 2007 at 12:14 am
Good idea, i’ll try to put a new version of this online in a few days with some examples of lists and maps as well.
There are a lot of other possible mappings that i haven’t explored yet, but if there are any other suggestions, i’ll gladly try to implement them.
July 24th, 2007 at 11:49 pm
That’s a cool diagram
September 5th, 2007 at 12:08 pm
Great job.
The only thing I would like to see is some comments in your code as I am new to ORM tools and C#.
Just a suggestion though!
Cheers
September 10th, 2007 at 4:54 pm
Thanks for the example diagrams and code. Small enough yet very useful as an sample.
One suggestion – you might want to add a copy of nunit.framework to the Lib folder to make the solution more self-contained.
September 10th, 2007 at 4:59 pm
Question – any particular reason you chose to let NHibernate use direct field access? Is there a performance advantage by doing so? Thanks.
September 10th, 2007 at 9:58 pm
thx for pointing out the nunit assembly issue… i’ve updated the zip file so it should be fixed now.
As for the direct field access… that’s just a habit to be honest. I prefer to let NHibernate use the fields directly so i’m free to put whatever code i want in my setters without having to worry about it being executed when NHibernate is initializing an object. Granted, in these examples it’s unnecessary. As for performance, i haven’t benchmarked it but the direct field access should actually be a little bit slower than using properties.
October 3rd, 2007 at 2:19 am
Great set of example mappings.
Do you write up your mapping and class files by hand or use some type of code generation? Personally, I think it would be nice to at minimum be able to generate the mapping files given one or more tables/views in a database from which to work with. Basic stubbing out of a class file based on each mapping file would also be nice to start out with.
If you have any suggestions, I’d be interested to know. Thanks again for your post.
- wg
October 3rd, 2007 at 8:44 am
I did those manually, but at work we do have a custom code generator which will generate the mapping files and the classes based on the database metadata. If you go that route, i would suggest making it possible to differentiate between generating the files initially, and being able to regenerate them constantly. In some cases, you will want to do something different than what was generated by default, so you should be able to modify the generated files without being worried that they will be overwritten the next time you re-generate.
October 6th, 2007 at 6:04 pm
Davy,
Looks cool! What is the custom code generator tool you use at work? I am looking for some tool to generate mapping files and the business entities.
Thanks
October 7th, 2007 at 10:28 am
Well i wrote the custom code generator for the client so unfortunately i can’t share it. Otherwise i would’ve put it online already
. You can also try the templates at http://www.mygenerationsoftware.com allthough i haven’t used MyGeneration yet so i don’t know if it’s any good. But it’s free so it won’t hurt to give it a shot.
October 13th, 2007 at 8:17 pm
I have two tables something like this :
Table : Account
AccountID
UserName
Password
Table : Address
AddressID
AccountID
Line1
Line2
State
My NHibernate mapping file looks like this :
For Table Account
For Table : Address
My Problem is it does entry in the Account table perfectly, but when it comes to do entry in Address Table it says AccountID can’t be null, I’m wondering shouldn’t it pick it up from the Account table as I’m passing in that object.
Is there something in the code need to be fixed ?
October 14th, 2007 at 11:36 am
your mapping files didn’t make it through apparantly so i can’t reallly say much about this…
October 15th, 2007 at 9:59 pm
When I test this solution, I get exeption: “Could not compile the mapping document: Northwind.Domain.Mappings.ProductCategory.hbm.xml”. Why? What can I do?
October 15th, 2007 at 10:12 pm
did you make any changes to the file? because it works over here
January 16th, 2008 at 4:18 pm
[...] NHibernate Mapping Examples « Davy Brion’s Blog [...]
May 18th, 2008 at 2:46 pm
Do you have some data for this new structure? or sql-script to transfer data from old northwind to northwind by u?
May 18th, 2008 at 2:57 pm
unfortunately i don’t…
July 8th, 2008 at 9:54 pm
[...] or Entity Framework or anything. You’ll need to use something else. In my example code I’m using Davy Brion’s NHibernate Northwind Sample as it’s a .NET 2.0 compiled solution and it’ll be acting as my [...]
July 22nd, 2008 at 8:04 pm
[...] I used NHibernate as my Data Layer recently when I got ASP.NET MVC running under .NET 2.0 using NHibernate examples from Davy Brion (who has an NHibernate Category on his [...]
August 31st, 2008 at 5:01 pm
[...] Note: this post was originally posted on July 15, 2007 [...]
September 3rd, 2008 at 6:39 pm
Very nice work, very helpful.
September 6th, 2008 at 12:49 am
Firstly, Greate post. Very useful.
I have a question i can google the answer! So i am hoping you may know.
I have a classic many-to-many situation in my database.
Table1(id|…) === Table3([id|table1.id|table2.id|sortOrder])=== Table2(id|…)
I have mapped this to my classes and this is working fine with each entity containing a collection of the other one.
however i have a column on my join table3 (sort order). And i dont know how to get hibernate to map this onto either entity.
Suggestion would be welcome. I hope i have explained the situation clearly enough.
September 6th, 2008 at 11:24 am
I don’t think you can do that… A true many-to-many only has two keys… once another column is added, it’s considered an own entity in its own right. In this case it might seem a bit drastic because of the column only contains a SortOrder, but i’m affraid you won’t be able to use a many-to-many association for that
I can be wrong though
October 27th, 2008 at 7:22 pm
JBurrow, probably too late of an answer, but I had to tackle the same thing (a many-to-many with an additional value). You just need to define the association table in one of the containing classes. For your example:
October 27th, 2008 at 7:25 pm
Well.. that didn’t work… (no xml posting… duhh). (why is there no preview?)
Without the xml, it’s:
class name=”Table1Class” table=”Table1″
set name=”Table3s” cascade=”save-update” table=”Table3″
key column=”Table1ID”
composite-element class=”Table3Class”
property name=”SortOrder”
many-to-one name=”Table2Class” column=”Table2ID”
/composite-element
/set
/class
January 10th, 2009 at 11:01 am
I’ve used the samples successfully! Thanks alot! How would I map a view that queries the Employees’ table, through to the Products’ table? An initial complaint from nhibernate is that it cannot find an id field.
March 22nd, 2009 at 4:35 pm
[...] original version of my NHibernate Mapping Examples was pretty popular, but they were based on NHibernate 1.2 and only showed some of the basics. I [...]
March 31st, 2009 at 4:41 pm
Beautiful.
My only complaint is that it could have done with some complex HQL examples.
“from Employees e where e.Orders.OrderLines.Discount > 5″
…and I know that won’t compile, because that demonstrates what I’m stuck trying to do. I was hoping your fantastic Northwind solution which give me the answer.
October 14th, 2009 at 7:56 pm
Hi..
I am new to NHIbernate.
Can anybody tellme how to do mapping / solve the current situation..
Product Table Category Table
productid categoryId
productname category
productdescription
category{string type}
Current Window: Product.Xaml [iam binding product entity to this form/window]
and my requirement is while accessing product entity from product table i need to access all categories from category table.So that it will be binded to a dropdownlist. is it possible? or i need to bind categories programmatically after loaded product entity ?
December 1st, 2009 at 7:37 pm
Great example!
Curious, did you use any tools for generating your mappings?
December 2nd, 2009 at 8:30 am
@Salman
nope, i don’t really believe in them
December 16th, 2009 at 8:16 pm
Davy,
In your example, you have a many-to-many relationship for territories and employees.
You used a Set in the Territory mapping file, and a bag in the employee mapping file.
Don’t you need to set one of them with the inverse=true attribute? To tell nhibernate which entity collection to use when their is a change?
December 16th, 2009 at 8:18 pm
ah ok, you only have the Territory entity that is able to add/remove employees, so its not bidirectional in that manner