0

I´m using Fluent Nhibernate with automapping and having problem setting up a bi-directional HasMany relationship because of my current inheritance.

I simplified version of my code looks like this

public abstract class BaseClass
{
    public BaseClass Parent { get; set; }
}

public class ClassA : BaseClass
{
    public IList<ClassB> BChilds { get; protected set; }
    public IList<ClassC> CChilds { get; protected set; }
}

public class ClassB : BaseClass
{
    public IList<ClassD> DChilds { get; protected set; }
}

public class ClassC : BaseClass
{
}

public class ClassD : BaseClass
{
}

Every class can have one parent and some parents can have childs of two types. I´m using table-per-type inheritance which result in the tables

  • "BaseClass"
  • "ClassA"
  • "ClassB"
  • "ClassC"
  • "ClassD"

To get a working bi-directional mapping I have made the following overrides (one example from ClassA)

mapping.HasMany<BaseType>(x => x.BChilds).KeyColumn("Parent_Id");
mapping.HasMany<BaseType>(x => x.CChilds).KeyColumn("Parent_Id");

This works fine on classes with only one type of children, but ClassA with two child types will get all subtypes of BaseType in each list which ofcourse will end up in an exception. I have looked at two different workarounds tho none of them feels really sufficient and I really believe there is a better way to solve it.

Workaround 1: Point to the concrete subtype in the HasMany mapping. (Updated with more info)

mapping.HasMany<ClassB>(x => x.BChilds).KeyColumns("Parent_Id");

(BaseType replaced with ClassB)

With this mapping NHibernate will in some cases look in the ClassB table for a column named Parent_Id, obviously there is no such column as it belongs to the BaseClass table. The problem only occurs if you add a statement based on BChilds during a ClassA select. e.g loading an entity of ClassA then calling ClassA.BChilds seems to work, but doing a query (using NhibernateLinq) something like 

Query<ClassA>().Where(c => c.BChilds.Count == 0)

the wrong table will be used. Therefore I have to manually create a new column in this table with the same name and copy all the values. It works but it´s risky and not flexible at all.

Workaround 2: Add a column to the BaseClass that tells the concrete type and add a where statement to the HasMany mapping.

(after my update to workaround1 I´m no longer sure if this could be a workable solution)

By adding a column they same way as it´s done when using table-per-hierarchy inheritance with a discriminatorValue. i.e BaseType table will get a new column with a value of ClassA, ClassB... Tho given how well NHibernate handles the inheritance overall and by reading the NHibernate manual I believe that the discriminator shouldn´t be needed in a table-per-type scenario, seems like Nhibernate already doing the hardpart and should be able to take care of this in a clean way to without adding a new column, just can´t figure out how.

user2420187
  • 16
  • 2

1 Answers1

0

What's your base class mapping and what does your subclass map look like? You should be able to do 

mapping.HasMany(x => x.BChilds);

And with the correct mapping, you shouldn't have a problem. If it's fluent nhibernate, look into 

UseUnionSubclassForInheritanceMapping();
Paul Connolly
  • 359
  • 2
  • 8
  • I´m using automapping so I don´t have any real classMaps beyond the mentioned overrides. `mapping.HasMany(x => x.BChilds); should be the same as` `mapping.HasMany(x => x.BChilds);` i.e. same as my workaround 1 (which is using the wrong table) I will look at UseUnionSubclassForInheritanceMapping(); – user2420187 May 26 '13 at 18:05
  • Looking at UseUnionSubclassForInheritanceMapping my first understanding is that you´ll use either that or the discriminator technique. I couldn´t notice any different by adding it to my mapping so I therefore suspect this is the strategy already in use. I will elaborate my question a little bit about when the problem with the wrong table occurs, something that slipped out of my mind after all the testing back and forth. Basically if go with the mapping above loading a ClassA object then call ClassA.BChilds will give the correct result, but a NhibernateLinq query will use the wrong table. – user2420187 May 26 '13 at 22:01