How does an entity get an ID before a transaction is committed in JPA/Play?

Question

See this question.

It turns out that even without committing the transaction manually, before the TX is committed, the person has an ID after calling the save() meth

Answer 1

as i know, we can't get the object's id (assume it's a auto-numbered) before it's persisted. and i personally think that it's quite dangerous to assign something which should be done by RDBMS outside it.