Mapping Generic Classes using NHibernate

Question

I'm trying to do the following, but it's complaining that the "classes referenced by 'extends' were not found". I think I need to having a mapping for each concrete type of Component but I can't specify the Attributes.Class twice..

The code is as follows:

[NHibernate.Mapping.Attributes.Class(Table = "Components", Abstract = true,
    NameType = typeof (Component<ContentItem>))]
public abstract class Component<T> : IComponent<T> where T : ContentItem
{
    ...
}

[NHibernate.Mapping.Attributes.JoinedSubclass(Table = "ComponentA", ExtendsType = typeof(Component<ItemA>))]
public class ComponentA : Component<ItemA>
{
    ...
}

[NHibernate.Mapping.Attributes.JoinedSubclass(Table = "ComponentB", ExtendsType = typeof(Component<ItemB>))]
public class ComponentB : Component<ItemB>
{
    ...
}

Where ItemA and ItemB inherit from ContentItem and are all mapped.

John Rayner · Accepted Answer · 2009-11-25 22:34:53Z

2

You can't map an open generic type like this, i.e. one that has an unspecified type parameter <T>. It just doesn't work.

Ayende discusses this in more detail on his blog.

answered Nov 25, 2009 at 22:34

John Rayner

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3 Comments

LucyB Over a year ago

If I follow that article and hard-code the information in the Component`1.hbm.xml file, then it appears to work fine. I just don't know how to do it using the Mapping.Attributes annotation. Annoyingly, T isn't actually mapped in the classes because it's not persisted by the database in this instance.

John Rayner Over a year ago

So it sounds like the persistent property of Component<T> do not actually depend on T. If this is the case, separate them into a ComponentBase (which is mapped), have Component<T> derive from ComponentBase (which is not mapped) and then have your Component<ItemA> class mapped as a joined-subclass of ComponentBase.

LucyB Over a year ago

Yes, that's what I ended up doing in the end. Thanks for your help.

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