[hibernate-dev] HHH-12125 - @GeneratedValue & 5.3

[Steve Ebersole]

The background is all in the Jira, but the crux is this... it is better to
allow a user to do this:

    @GeneratedValue( strategy=SEQUENCE, generator="my_seq")

rather than:

    @GeneratedValue( strategy=SEQUENCE, generator="my_seq")
    @SequenceGenerator( name="my_seq", sequenceName="my_seq" )

You can see that `SequenceGenerator` is completely unnecessary in this case
because it adds no new information beyond what is already available on the
`@GeneratedValue`.

This works great for `GeneratedValue#strategy=SEQUENCE` and
`GeneratedValue#strategy=TABLE`, however consider this mapping:

    @GeneratedValue( strategy=AUTO, generator="increment" )
    @GenericGenerator( name = "increment", strategy = "increment" )

Here we have the same underlying concern - the `@GenericGenerator` is just
noise, it adds no additional information.  In keeping with the work done as
part of HHH-12125 it would be great to allow users to instead just say:

    @GeneratedValue( strategy=AUTO, generator="increment" )

The problem here is that this last one is actually the responsibility of
`org.hibernate.boot.model.IdGeneratorStrategyInterpreter#determineGeneratorName`
to interpret, but it is not passed the `GeneratedValue#generator` value.

So the easiest solution would be to add an additional parameter to
`IdGeneratorStrategyInterpreter#determineGeneratorName` for passing in the
generator name.  The concern here is that `IdGeneratorStrategyInterpreter`
is defined in the API space and could very well have custom impls.

A lesser solution wold be to add checks to the code that calls
`IdGeneratorStrategyInterpreter#determineGeneratorName` to check for
"magic" generator names before asking the
`IdGeneratorStrategyInterpreter`.  This is just a very inflexible and
non-extensible solution, but maybe this works for 5.3 and we can adjust
`IdGeneratorStrategyInterpreter` as part of 6.0.

Thoughts?