Introduce the Specification<T> interface for find() and findOrCreate()

[arteymix]

We have methods in BaseDao that expects a template of an entity as input and attempt to find a persistent entity in the database that satisfy some kind of business key. However, with the find(T) signature, this is unclear. We replace those by a find(Specification<T>) signature.

This will allow us in the future to cleanup BusinessKey by having some kind of standard way of generating HQL clauses and Criterions.

[arteymix]

It would be nice to have implementations of Specification to fetch entities by ID and business key.

• Specifications.byId(Long)
• Specifications.byBusinessKey(T)
• Specifications.byIdentifiable() would become ID if non-null else some business key

Find or create expects a little more than an ID/business key, we could have a special sub-interface for supplying a template object to create if not found.

interface SpecificationWithTemplate<T> extends Specification<T> {
    T createEntity();
}

Specification.byIdentifiable() would return a SpecificationWithTemplate where the template is the passed identifiable.

Hibernate has support for defining natural ID which are essentially business key. We could move all the BusinessKey declarations in the model declaration and use natural ID under the hood for business key-based specification.

[arteymix]

To be valid, a specification must have either an ID or a complete business key.