The error occurs because the object’s ID is set. Hibernate distinguishes between transient and detached objects and persist
works only with transient objects. If persist
concludes the object is detached (which it will because the ID is set), it will return the “detached object passed to persist” error. You can find more details here and here.
However, this only applies if you have specified the primary key to be auto-generated: if the field is configured to always be set manually, then your code works.