[Hibernate] New features: Hibernate ODMG + collection filters

[Gavin K. <ga...@ap...>]

Hi everyone. I've created an experimental ODMG3 API for Hibernate.
Unfortunately I don't own a copy of the ODMG spec (you have to *buy* it,
unfortunately) so I'm not 100% sure of some semantics. Also, some concepts
don't map perfectly to an O/R tool.

The main missing thing is an ODMG-style OQL. I don't think Hibernate's query
language is very far removed from OQL but there are some differences
(particularly collections). I would need some proper OQL documentation
before we could start to support the ODMG style syntax because the ODMG
implementations I've investigated don't seem agree on some of these points.

Another issue is persistence by reachability. Mapping all associations with
cascade="save/update" (also a new feature) would come close to the ODMG
programming model but perhaps we should allow some setting which forces
cascade to *default* to "save/update".

Hibernate collection wrappers now implement the ODMG interfaces, ie
DCollection, DMap, DList, DSet, etc. To do this, I needed
collection-filtering functionality. I saw that this was useful for Hibernate
itself, since it lets you take a large, uninitialized collection and load up
only the elements you are interested in. Accordingly Ive added filtering
methods to the Session interface like:

   filteredCollection = session.filter(collection, query)

A filter is just a query where you can refer to "this", the collection
element. So, for example:

  blackCats = session.filter( cat.getKittens(), "where this.color='black'
order by this.name" );
  rivals = session.filter( cat.getPotentialMates(), "select this.mate" );

Now, I certainly don't propose to make the ODMG API the recommended way to
use Hibernate. Hibernate's own API is much more expressive. It would be
nice, though, if we could find someone here with a better knowledge of the
ODMG spec to take ownership of this feature and develop it towards fuller
compliance.

Gavin