Question from a colleague:

> > > Can anyone point me to an example of a mapping file which illustrates how to
> > > model a collection of instances of an abstract class?
> > >
> > > The excellent article on the visitor pattern discusses the java side in detail,
> > > see http://www.hibernate.org/280.html, but I would like to know what the
> > > mapping of a collection of polymorphic objects would look like.
> > > For example, if I had a class called Congress I would like to add an attribute
> > > 'members' which contains a list of Politian objects. In the model I don't
> > > care whether they are Democrat, Republic or just Lazy Politiions. That is a
> > > run time issue. I will probably be using the one relation per concrete class
> > > approach for my inheritance model. So I need a way to tell Hibernate how to
> > > gather selections from several differnent tables into a single list that can
> > > be processed by applying abstract methods.
> > >
> > > If the answer is the manual please just point me to the relevant page.

Regards,

You mean table-per-subclass?


If you want table-per-subclass, try this --

Schema:

where REP_ID and DEM_ID are also foreign key references to P_ID.

Mapping:

The <joined-subclass> tag maps a subclass to a new table. All props declared in the subclass will be in this table. A Republican object lookup will require a join of both tables (POLITICIAN + REPUBLICAN) based on the IDs.

OK I can see I have not made it completely clear what I am trying to do.
I can see that I will need some tables to represent the base class and
the derived classes and that using the joined-subclass construct as you
have suggested will solve that part of the problem nicely. So thanks for
that example:- it is useful. However, to complete the story I am looking
for an example that shows what would be required to generate a selection
that retrieves a set of various polititians (Democrat, Republican etc).
In particular, I would like to see how Hibernate supports the scenario of
adding a few more types of polititian, e.g. Green, Idle, and Looney.
A typical application would be written to use the generic Politition
interface to these classes and would need very little alteration outside
the DB layer.

Here are two examples which might occur in the same application:-

1. A government is made up of politians and exists for a period of time:-

class Gov
{
ID id;
Politian[] main_party;
Politian[] opposition;
Date start_date;
Date end_date;
}

How should I generate a method which will return a list of the politions in
the opposition, which may include more than one class of polition?

2. We may want to record how each politian in a government voted on a particular act:-

class Vote
{
ID id;
String act;
Politian[] voted_for;
Politian[] voted_against;
Politian[] abstained;
}

How should I write the mapping so that I can get a list of Politians that
abstained from voting in a given act? (So that I can call a method such as
MarkAsUseless()).

I hope this makes what I am trying to do more lucid without introducing to
many terms from UK politics to confuse matters.