为什么我保存就是不行，insert student的时候，总是报外键的错误，可是前面前面明明有insert grade的语句啊？

public static void addStudentsBak(){

        Configuration config=new Configuration().configure();

        SessionFactory sessionFactory = config.buildSessionFactory();

        Session session = sessionFactory.openSession();

        Transaction transaction = session.beginTransaction();

//创建班级对象

Grade g=new Grade("quanyecha","quanyechaban");

//创建学生对象

Student s1=new Student("jiegeng","女","12345678998");

Student s2=new Student("gewei","女","98765432112");

Set sSet=new HashSet<Student>();

//一对多，学生对象添加到班级对象中

sSet.add(s1);

sSet.add(s2);

g.setStudents(sSet);

//g.getStudents().add(s1);

//g.getStudents().add(s2);

//保存班级对象

session.save(g);

//保存学生对象

session.save(s1);

session.save(s2);

transaction.commit();

//关闭session

session.close();

sessionFactory.close();

}