[ http://jira.nhibernate.org/browse/NH-662?page=3Dcomments#action_1452=
1 ]
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mark lewman commented on NH-662:
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> Class Hierarchies
> -----------------
>
> Key: NH-662
> URL: http://jira.nhibernate.org/browse/NH-662
> Project: NHibernate
> Type: New Feature
> Components: Core
> Versions: 1.2.0.Beta2
> Reporter: Anton Gogolev
> Priority: Minor
> Fix For: LATER
>
> NH can only persist/load classes explicitly mapped to database tables in =
HBM files. On the one hand, this is quite logical. But on the other it limi=
ts further use of mapped classes. For instance, it is impossible to inherit=
from them and save the ancestor since its type is not mentioned anywhere i=
n the mapping files. Here's an example:
> // Base is mapped to, say, Base table in Base.hbm.xml
> class Base
> {
> public int A
> { get ...; set ... }
> protected virtual Foo()
> {
> // Some logic.
> }
> }
> // Derived has no mapping file
> class Derived : Base
> {
> protected override Foo()
> {
> // Some other logic
> }
> }
> static void Main()
> {
> ISession session =3D sessionFactory.OpenSession();
> Base base =3D new Base();
> base.A =3D 12;
> session.Save(base); // This succeeds
> base =3D session.Load(typeof(Base), 2) as Base; // This succeeds as w=
ell
> base =3D new Derived(); // Note "Derived" here
> base.A =3D 13;
> session.Save(base); // And this fails
> // We won't even get here, but if we did,
> // this would have failed too
> base =3D session.Load(typeof(Derived), 3) as Derived;
> }
> As you can see, the logic behind this is quite OOP-ish :) You inherit fro=
m some class to extend its functionality and state (in other words, functio=
ns and member variables). However, this is not possible with NHibernate, si=
nce it requires every derived class to be mapped somehow.
> How I see it done (conceptually) is as follows.
> Upon persisting an entity NH should traverse a list of base classes of th=
e entity being persisted, until it finds a type which is mapped to a databa=
se table. This type is used to persist an entity.
> When loading an entity from the database, NH can freely create an instanc=
e of derived class, since it is given its type (typeof(Derived) in the exam=
ple above). Then again it traveses a list of base classes, and loads derive=
d object as an object of base class. Of course some properties remain unini=
tialized, but that's not a problem, I think.=20
--=20
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