[hibernate-dev] Hibernate as a persistence engine

[Adam Warski]

Hello,

I took a look at the new meta-model proposed by Steve; in case anybody hasn't seen it it's here:
http://anonsvn.jboss.org/repos/hibernate/sandbox/trunk/new-metadata/

If I correctly understood Steve and the code, the model builds on the JPA2 meta-model. In the proposition there are three levels of meta-data:
1) database
2) mapping
3) java

However as Steve mentioned on IRC, there is a problem with alternate entity modes (today these are "map" and "xml").
(For me they are very useful, as Envers uses the "map" entity mode to dynamically generate the audit entities.)

Don't you think that an "entity" meta-data level would be useful?
The basic layers would then be:
1) database - information about tables, columns, foreign keys, indexes
2) mapping - binding between levels 1 and 3, that is, assignment from properties to columns, from entities to tables etc
3) entity - information about entity names, property names, property types (a property can be a simple value, many-to-one, one-to-many, (...) relation, a component etc). For example, a many-to-many relation property would have no idea that it's mapped using a join table.

Now there could be several layers that build on that. Each such layer would have to specify what is an entity and what is a property; how to get/set a property from an entity; how to read other meta-data (from annotations? or from xml?)
4a) java - an entity is a class; holds information if properties are accessed using fields or getters/setters
4b) map - an entity is a map; properties accessed using Map.get()
4c) xml

In the longer run, maybe Hibernate could become more of a "persistence engine".
That is, it would provide a nice entity abstraction of a database (so abstracting level 1 using level 3 as the access point). 
It would play nicely with another project that I have in mind, namely an ORM for Scala that would use Hibernate. Only a new entity mode (level 4) would have to be written, which would be a translation from Scala constructs to entities, properties, relations. (Maybe that would even be something that Bob could use in TorqueBox.)

What do you think?

-- 
Merry Christmas! :)
Adam