一对多 annotation 保存

场景:hibernate annotation 一对多保存对象时不能插入外键解决思路

hibernate annotation 一对多保存对象时不能插入外键
各位大侠,请帮帮小弟吧:在做一对多保存对象时出现不能插入外键的现象,这怎么解决,
Java code
@Entity
@Table(name="TEACHERS")
@SequenceGenerator(name="SEQ_TEACHERS",sequenceName="SEQ_TEACHERS")
public class Teacher {

    
    private Long id;
    
    private String code;
    
    private String name;
    
    private String type;
    
    private List<Student> students;
    
    @Id
    @GeneratedValue(generator="SEQ_TEACHERS",strategy=GenerationType.SEQUENCE)
    @Column(name="id")
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    
    @Column(name="CODE")
    public String getCode() {
        return code;
    }
    public void setCode(String code) {
        this.code = code;
    }
    
    @Column(name="NAME")
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    
    @Column(name="TYPE")
    public String getType() {
        return type;
    }
    public void setType(String type) {
        this.type = type;
    }
    
    @OneToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER)
    @JoinColumn(name="TECH_ID",nullable=true,updatable=true)
    public List<Student> getStudents() {
        return students;
    }
    public void setStudents(List<Student> students) {
        this.students = students;
    }
}


Java code

@Entity
@Table(name="STUDENTS")
@SequenceGenerator(name="SEQ_STUDENTS",sequenceName="SEQ_STUDENTS")
public class Student {

    
    private Long id;
    
    private String code;
    
    private String name;
//    @Column(name="TECH_ID")
//    private Long techId;
    
    private Teacher teacher;
    
    @Id
    @GeneratedValue(generator="SEQ_STUDENTS",strategy=GenerationType.SEQUENCE)
    @Column(name="id")
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    
    @Column(name="CODE")
    public String getCode() {
        return code;
    }
    public void setCode(String code) {
        this.code = code;
    }
    
    @Column(name="NAME")
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    
//    public Long getTechId() {
//        return techId;
//    }
//    public void setTechId(Long techId) {
//        this.techId = techId;
//    }
    
    @ManyToOne(cascade=CascadeType.ALL)
    @JoinColumn(name="TECH_ID")
    public Teacher getTeacher() {
        return teacher;
    }
    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }
}


Java code

public class AssociationsTest {

    /**
     * @param args
     */
    public static void main(String[] args) {
//        SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
        SessionFactory sessionFactory = new AnnotationConfiguration().configure().buildSessionFactory();
        Session session = sessionFactory.openSession();
        Transaction trans = session.beginTransaction();
        Teacher t = new Teacher();
        t.setCode("000001");
        t.setName("xiaoxue");
        t.setType("shuxue");
        List<Student> list = new ArrayList<Student>();
        Student s0 = new Student();
        s0.setCode("000001");
        s0.setName("xioali");
        list.add(s0);
        Student s1 = new Student();
        s1.setCode("000002");
        s1.setName("zhangsan");
        list.add(s1);
        t.setStudents(list);
        session.save(t);
        trans.commit();

    }

}


执行到session.save(t);时,报如下错误:
Exception in thread "main" org.hibernate.exception.ConstraintViolationException: Could not execute JDBC batch update
at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:71)
at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:43)
at org.hibernate.jdbc.AbstractBatcher.executeBatch(AbstractBatcher.java:253)
at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:237)
at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:141)
at org.hibernate.event.def.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:298)
at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:27)
at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1000)
at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:338)
at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:106)
at com.myHibernate.test.AssociationsTest.main(AssociationsTest.java:39)
Caused by: java.sql.BatchUpdateException: ORA-01400: 无法将 NULL 插入 ("SYSTEM"."STUDENTS"."TECH_ID")

at oracle.jdbc.driver.DatabaseError.throwBatchUpdateException(DatabaseError.java:367)
at oracle.jdbc.driver.OraclePreparedStatement.executeBatch(OraclePreparedStatement.java:8739)
at com.mchange.v2.c3p0.impl.NewProxyPreparedStatement.executeBatch(NewProxyPreparedStatement.java:1723)
at org.hibernate.jdbc.BatchingBatcher.doExecuteBatch(BatchingBatcher.java:48)
at org.hibernate.jdbc.AbstractBatcher.executeBatch(AbstractBatcher.java:246)
... 8 more

------解决方案--------------------
你设了级联,你在保存一这方,多的那方也会保存,
但是多的那方你要保证每个字段都有值,或可以为空,或者有默认值。
比如
a 表 e,f,d 三个字段,
b, 表有 x,y,e,e是引用a的e做外键,
你插人a时,
b,也会插,这是b表里只有e这个字段有值,x,y是空的,如果没有默认值,后值不允许为空,
这时 b表就会报错