Question

Suppose I have three classes. It is valid to instantiate A, but there are also special cases B and D which subclass A, adding additional information.

How would I do the mapping files for this in (fluent) NHibernate?

public class A {     public int ID { get; set;}     public string CommonProperty1 { get; set; }     public string CommonProperty2 { get; set; } }  public class B : A {     public string BSpecificProperty1 { get; set; } //not null     public string BSpecificProperty2 { get; set; } //not null }  public class D : A {     public string DSpecificProperty { get; set; } //not null } 

I tried the following, but it doesn’t work at all:

public class AMap : ClassMap<A> {     public AMap()     {         Id(x => x.ID);          Map(x => x.CommonProperty1);         Map(x => x.CommonProperty2);     } }  public class BMap : ClassMap<B> {     public BMap()     {         References(x => x.ID);         Map(x => x.BSpecificProperty1)             .CanNotBeNull();         Map(x => x.BSpecificProperty2)             .CanNotBeNull();     } }  public class DMap : ClassMap<D> {     public DMap()     {         References(x => x.ID);          Map(x => x.DSpecificProperty)             .CanNotBeNull();     } } 

Answer 1

I’m not sure I understand what you mean by ‘map a subclass one-to-one’, but if you want to map inheritance where the subclasses have properties that are not nullable, you can do like this in Fluent-NHibernate:

// Domain classes public class Animal {     public virtual int Id { get; set; }     public virtual string Name { get; set; } }  public class Cat : Animal {     public virtual int WhiskerLength { get; set; }     public virtual int ClawCount { get; set; } }  public class Dog : Animal {     public virtual int TailWagRate { get; set; } }    // Mapping file public class AnimalMap : ClassMap<Animal> {     public AnimalMap()     {         Id(x => x.Id)             .WithUnsavedValue(0)             .GeneratedBy.Native();          Map(x => x.Name);          var catMap = JoinedSubClass<Cat>('CatId', sm => sm.Map(x => x.Id));          catMap.Map(x => x.WhiskerLength)             .CanNotBeNull();         catMap.Map(x => x.ClawCount)             .CanNotBeNull();          JoinedSubClass<Dog>('DogId', sm => sm.Map(x => x.Id))             .Map(x => x.TailWagRate)                 .CanNotBeNull();     } } 

Since you want the subclasses’ properties to be not-null, you have to use the table-per-class (joined-subclass) way of modeling the inheritance. This is because table-per-hierarchy requires all subclass properties to be nullable.

I hope it helps.

/Erik