@Entity
class A {
    //...
}


@Entity
class B {
    @OneToMany
    List<A> list = new ArrayList<>();
}

//функция обеспеч. постоянство
@PersistenceContext(unitName = "db0")
EntityManager em;
EntityTransaction tx = em.getTransaction();
A a = new A():
B b = new B();

tx.begin();
//..
tx.commit();