我无法编辑记录合并

Question

我在JPA上使用合并时无法编辑记录

@Override
public void merge(int id , String nom) {
  EntityTransaction et = em.getTransaction();
  et.begin();
  Grade  grade=  em.find(Grade.class,id);
  Grade gr=new Grade();
  gr.setNumGrd(grade.getNumGrd());
  gr.setNomGrd(nom);
  em.merge(gr);
  et.commit();
}