Java 弹簧休眠保存实体与onetomany失败

我在一个使用springmvc和hibernate的项目中工作，我在模型中有3个实体，代码如下：

Ciclos.java

@Entity
@Table(name="ciclos") 
public class Ciclos implements Serializable{

    private static final long serialVersionUID = 1L;

    @Id
    @Column(name = "id")
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;

    @Column(name = "nombre")
    @NotEmpty(message="{string.empty}")
    @Size(min=5, max=55, message="{string.invalid.size}") 
    private String nombre;

    @Column(name = "fecha_inicial")
    @NotNull(message="{date.invalid}")
    private Date fechaInicial;

    @Column(name = "fecha_final")
    @NotNull(message="{date.invalid}")
    private Date fechaFinal;

    @OneToMany(cascade=CascadeType.ALL, fetch = FetchType.EAGER)
    @Fetch(value = FetchMode.SELECT)
    private List<Niveles> niveles;
    .....

现在，假设我有一个ciclo Ciclos，它拥有一个Nivel，Nivel拥有两个grados。每个实体都是全新的，还没有保存在DB上，我想保存所有的只是保存Ciclos enity，我发送给我的daoimpl以使用这个方法

@Transactional(readOnly=false)
public Ciclos saveCiclo(Ciclos ciclo) {
    return em.merge(ciclo);
}

但它总是抛出：

Hibernate: select ciclos0_.id as id1_0_1_, ciclos0_.fecha_final as fecha_fi2_0_1_, ciclos0_.fecha_inicial as fecha_in3_0_1_,ciclos0_.nombre as nombre4_0_1_, niveles1_.ciclos_id as ciclos_i1_0_3_, niveles2_.id as niveles_2_1_3_, niveles2_.id as id1_3_0_, niveles2_.ciclo_id as ciclo_id3_3_0_, niveles2_.nombre as nombre2_3_0_ from ciclos ciclos0_ left outer join ciclos_niveles niveles1_ on ciclos0_.id=niveles1_.ciclos_id left outer join niveles niveles2_ on niveles1_.niveles_id=niveles2_.id where ciclos0_.id=?
Hibernate: insert into ciclos (fecha_final, fecha_inicial, nombre) values (?, ?, ?) Hibernate: insert into niveles (ciclo_id, nombre) values (?, ?) Hibernate: insert into grados (grado, nivel_id) values (?, ?) 
Hibernate: insert into grados (grado, nivel_id) values (?, ?)
Hibernate: insert into ciclos_niveles (ciclos_id, niveles_id) values (?, ?) 
Hibernate: insert into ciclos_niveles (ciclos_id, niveles_id) values (?, ?)   
org.springframework.dao.DataIntegrityViolationException: could not execute statement; SQL [n/a]; constraint [null]; nested exception is org.hibernate.exception.ConstraintViolationException: could not execute statement   at
org.springframework.orm.jpa.vendor.HibernateJpaDialect.convertHibernateAccessException(HibernateJpaDialect.java:248) at
org.springframework.orm.jpa.vendor.HibernateJpaDialect.translateExceptionIfPossible(HibernateJpaDialect.java:214) at
org.springframework.orm.jpa.JpaTransactionManager.doCommit(JpaTransactionManager.java:521)

---

有什么问题吗？

根据需要，您需要为双向关系提供mappedBy元素。

谢谢，这正是我需要的

@Transactional(readOnly=false)
public Ciclos saveCiclo(Ciclos ciclo) {
    return em.merge(ciclo);
}

Hibernate: select ciclos0_.id as id1_0_1_, ciclos0_.fecha_final as fecha_fi2_0_1_, ciclos0_.fecha_inicial as fecha_in3_0_1_,ciclos0_.nombre as nombre4_0_1_, niveles1_.ciclos_id as ciclos_i1_0_3_, niveles2_.id as niveles_2_1_3_, niveles2_.id as id1_3_0_, niveles2_.ciclo_id as ciclo_id3_3_0_, niveles2_.nombre as nombre2_3_0_ from ciclos ciclos0_ left outer join ciclos_niveles niveles1_ on ciclos0_.id=niveles1_.ciclos_id left outer join niveles niveles2_ on niveles1_.niveles_id=niveles2_.id where ciclos0_.id=?
Hibernate: insert into ciclos (fecha_final, fecha_inicial, nombre) values (?, ?, ?) Hibernate: insert into niveles (ciclo_id, nombre) values (?, ?) Hibernate: insert into grados (grado, nivel_id) values (?, ?) 
Hibernate: insert into grados (grado, nivel_id) values (?, ?)
Hibernate: insert into ciclos_niveles (ciclos_id, niveles_id) values (?, ?) 
Hibernate: insert into ciclos_niveles (ciclos_id, niveles_id) values (?, ?)   
org.springframework.dao.DataIntegrityViolationException: could not execute statement; SQL [n/a]; constraint [null]; nested exception is org.hibernate.exception.ConstraintViolationException: could not execute statement   at
org.springframework.orm.jpa.vendor.HibernateJpaDialect.convertHibernateAccessException(HibernateJpaDialect.java:248) at
org.springframework.orm.jpa.vendor.HibernateJpaDialect.translateExceptionIfPossible(HibernateJpaDialect.java:214) at
org.springframework.orm.jpa.JpaTransactionManager.doCommit(JpaTransactionManager.java:521)