Question

I started using NHibernate today, but I cannot figure out how I setup a simple relation between two tables. I don’t really know what it’s called, it could be one-to-many or foreign key relation (I’m not that into database design and the terms used), but here’s a very simple example.

I have a table Product with attributes Id (PK), ProductName and CategoryId. Then I have a table Categories with attributes Id (PK) and CategoryName.

I created these classes:

public class Product
{
    public virtual int Id { get; set; }  
    public virtual string ProductName { get; set; }  
    public virtual int CategoryId { get; set; }  

    public virtual Category Category { get; set; }  

    public virtual string CategoryName  
    {  
        get { return this.Category == null ? String.Empty : this.Category.CategoryName; } 
    }
}

public class Category  
{  
    public virtual int Id { get; set; }
    public virtual string CategoryName { get; set; }
}

In other words, I simply want the Product to store to which category it belongs (via the CategoryId attribute which points to an Id in the Categories table). I don’t need the Category class to hold a list of related Products, if that makes it any simpler.

To make it even more clear what I’m after, this is the SQL that I’m expecting:

SELECT Products.*, Categories.*
FROM Products INNER JOIN Categories ON Products.CategoryId = Categories.Id

at least that’s what I think it should be (again, I’m not that good at database design or queries).

I can’t figure out which kind of mapping I need for this. I suppose I need to map it in the Product.hbm.xml file. But do I map the CategoryId as well? And how do I map the Category property?

It seems like I would need a ‘one-to-many’ relation since I have ONE category per product (or is this reasoning backward?) but it seems like there is no one-to-many mapping…

Thanks for any help!

Addition:

I tried to add the many-to-one relation in the Person mapping, but I keep getting an exception saying “Creating proxy failed”, and in the inner exception “Ambiguous match found”.

I should maybe mention I am using an old version of NHibernate (1.2 I think) because that is the only one I got running with MS Access due to it not finding the JetDriver in newer versions.

I’ve put the mapping files, classes, and code where the error occurs in screenshots because I can’t figure out how to post XML code here… It keeps reading it as html tags and skipping half of it. Anyway.

The mappings:
http://www.nickthissen.nl/Images/tmp7B5A.png

The classes:
http://www.nickthissen.nl/Images/tmpF809.png

The loading code where the error occurs:
http://www.nickthissen.nl/Images/tmp46B6.png
(As I said, the inner exception says “Ambiguous match found”.

(Product in my example has been replaced by Person)

The Person and Category classes inherit Entity which is an abstract base class and defines the Id, Deleted, CreatedTime and UpdatedTime properties.
The code where the error occurs is in a generic ‘manager’ class (type parameter TEntity which must inherit Entity). It is simply supposed to load all entities with the Deleted attribute false. In this case, TEntity is ‘Person’.

It works fine if I leave out the many-to-one Category mapping in the Person mapping, but then obviously the Category property is always null.

Oh yeah, sorry about the mix between C# and VB, the C# code is in a generic framework I use for multiple projects while the VB part is the actual implementation of that framework on my website and I just happened to use VB for that.

Help? Thanks!

Answer 1

In your Product class only needs to contain a Category object, you don’t need a CategoryId property. Then in your Product mapping you need to have this entry

<many-to-one name="Category" column="CategoryId" />

UPDATE:
Your mappings appear to be missing the fully qualified name of the mapped class in the tag. See http://nhibernate.info/doc/nh/en/index.html#mapping-declaration-class

UPDATE 2:

See if this helps you NHibernate 1.2 in a .NET 4.0 solution