Question

I have a very extensible and modular application. I want to extend mapped Entites from other assemblies. Yet, I still need to operate on the base classes.

Example:

Parent class mapping in Assembly A:

    public class PersonMap : ClassMap<Person> 
    {
        public PersonMap()
        {
            Table("Persons");

            Id(x => x.Id).GeneratedBy.Assigned();
        }
}

Child class mapping in Assembly B:

public class EmployeeMap : SubclassMap<Employee>
{
    public EmployeeMap()
    {
        Table("Persons");
        Extends(typeof(Person));

        KeyColumn("Id");

        LazyLoad();

        HasMany<Assignments>(x => x.Assignments).KeyColumn("Id").Inverse().Cascade.AllDeleteOrphan().LazyLoad().NotFound.Ignore();
    }
}

Now whenever I create a Person in some code of Assembly A it is saved as Employee by NHibernate. And that leads to a class cast exception due to proxying whenever I save a Person and try to refresh it in Assembly A. Assembly A must not have a dependency on Assembly B.

I need to operate on the parent class in all methods of assembly A. The child class is only used in other assemblies.

How can I map something like that? How can I tell NHibernate to just save it as the parent class? I use SaveOrUpdate to persist the entities; How can I correctly extend entities and yet save them to the same table without discriminator? Can’t NHibernate differentiate by object type? Is there a workaround?

I do not want to specify manual proxies as I’d have to create a proxy for every entity! I cannot use the visitor pattern due to dependency problems.

I need a way to extend a mapped entity in a different assembly without such problems! The database is legacy, I cannot change it. How would you work around the issue?

Answer 1

Solved it by using a HasOne Mapping on the same Table and not using a subclass. This does not produce ideal code but is reather free of problems.