6.0 persister redesign

See also the https://github.com/hibernate/hibernate-orm/wiki/6.0---Type-redesign page. A further aspect of the Type redesign is redesigning the persister contracts.

ORM meeting Monday to start discussing this work...

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Initial thoughts...

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The idea of persister (EntityPersister and CollectionPersister) has always been 2-fold:

• Metadata about the thing (entity/collection)

• Persistence-related calls for the thing

Some goals for this work include:

• Redesign the metadata aspect. I say redesign, but really the initial "design" grew organically with no (little) up front plan/design

• Defining a new "persister" for composites/embeddables
• Decide about splitting these concerns or leaving them combined
• Decide what to do with the persistence aspect.

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In terms of the metadata aspect, I'd like to align with the JPA design and the SQM design (which is heavily influenced by the JPA design). Not because I think the JPA design is any better (in fact it is not as good in some ways), but more because (1) it is well understood and (2) represents a least-common-denominator as far as its design focus.

I'd also like to take some of the ideas originally developed in the metamodel branch work wrt hierarchy modeling.

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In JPA EntityType models what we would historically call the persister, whereas the historical Hibernate EntityType would model a specific use of that entity (a one-to-one, many-to-one, any, etc). This is the "least-common-denominator" aspect discussed above. We obviously ultimately need both. The metadata about a specific usage would be relative to a (Singular)Attribute. I think that is the most appropriate "mental model" given all requirements.