获取主键，而不是在一对多的关系

整个对象，我有以下类别：获取主键，而不是在一对多的关系

@Entity 
@Table(name = "elements") 
@Inheritance(strategy=InheritanceType.JOINED) 
@XmlRootElement 
public abstract class Elements implements Serializable { 

    private static final long serialVersionUID = 1L; 

    @Id 
    @GeneratedValue(strategy = GenerationType.IDENTITY) 
    @Basic(optional = false) 
    @Column(name = "idelement") 
    private Integer idElement; 

    @Basic(optional = false) 
    @Column(name = "code") 
    private String code; 

    @OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL, mappedBy = "idElement") 
    @Fetch(value = FetchMode.SUBSELECT) 
    private Collection<Alarms> alarmsCollection; 

    @OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL, mappedBy = "idElement") 
    @Fetch(value = FetchMode.SUBSELECT) 
    private Collection<ElementsHistorical> elementsHistoricalCollection; 

    @ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.PERSIST, mappedBy = "elementsCollection") 
    @Fetch(value = FetchMode.SUBSELECT) 
    private Collection<ElementsGroups> elementsGroupsCollection; 


    //Constructors, getters and setters 
} 

@Entity 
@Table(name = "alarms") 
@XmlRootElement(name = "Alarms") 
public class Alarms implements Serializable { 
    private static final long serialVersionUID = 1L; 

    @Id 
    @GeneratedValue(strategy = GenerationType.IDENTITY) 
    @Basic(optional = false) 
    @Column(name = "idalarm") 
    private Integer idAlarm; 

    @JoinColumn(name = "idelement", referencedColumnName = "idelement") 
    @ManyToOne(optional = false) 
    private Elements idElement; 

    //Constructors, getters and setters 

}

我创建了一个Web服务的球衣与CRUD操作的DAO类治疗数据。通过这个实现，当我调用GET方法时，我得到之内的整个Elements对象。这就是所谓的DAO方法：

public List<Alarms> getAlarms(){ 
    Session session = SessionUtil.getSession();  
    Query query = session.createQuery("from Alarms"); 
    List<Alarms> alarms = query.list(); 
    session.close(); 
    return alarms; 
}

我不希望这fetch = FetchType.EAGER提取类型，因为我只需要的Elements的PK，但是经过一番研究，这是我发现让我的服务工作的唯一办法。我试过this和this的方法，但我一直无法使它工作。

来源

2017-03-03 Juan

您可以在查询中使用左外连接提取。

Query query = session.createQuery("from Alarms alarm left outer join fetch alarm.idElement element");

现在你可以保持fetch = FetchType.LAZY（这是默认值）。

来源

2017-03-03 11:00:56

它没有工作，返回的结果相同 – Juan

前取不急于类似，它只是另一种方式来获得渴望 –

是的数据，但在这种情况下，你只有当你需要得到相关的对象。不是每次。 –

尝试将@XmlTransient注释添加到您要忽略的Elements类中的字段。这样的：

@Entity 
@Table(name = "elements") 
@Inheritance(strategy=InheritanceType.JOINED) 
@XmlRootElement 
public abstract class Elements implements Serializable { 

    private static final long serialVersionUID = 1L; 

    @Id 
    @GeneratedValue(strategy = GenerationType.IDENTITY) 
    @Basic(optional = false) 
    @Column(name = "idelement") 
    private Integer idElement; 

    @Basic(optional = false) 
    @Column(name = "code") 
    @XmlTransient 
    private String code; 

    @OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL, mappedBy = "idElement") 
    @Fetch(value = FetchMode.SUBSELECT) 
    @XmlTransient 
    private Collection<Alarms> alarmsCollection; 

    @OneToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL, mappedBy = "idElement") 
    @Fetch(value = FetchMode.SUBSELECT) 
    @XmlTransient 
    private Collection<ElementsHistorical> elementsHistoricalCollection; 

    @ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.PERSIST, mappedBy = "elementsCollection") 
    @Fetch(value = FetchMode.SUBSELECT) 
    @XmlTransient 
    private Collection<ElementsGroups> elementsGroupsCollection; 


    //Constructors, getters and setters 
}

来源

2017-03-03 12:01:25

您可以使用多选与Tuple，只能选择特定的列。

CriteriaBuilder cb = entityManager.getCriteriaBuilder(); 
    CriteriaQuery<Tuple> cq = cb.createTupleQuery(); 
    Root<Alarms> root = cq.from(Alarms.class); 
    CollectionJoin<Object, Object> joinCollection = root.joinCollection("alarmsCollection"); 
    cq.multiselect(joinCollection.get("idElement")); 
    List<Tuple> tupleResult = entityManager.createQuery(cq).getResultList(); 
    for (Tuple t : tupleResult) { 
     Long idElement = (Long) t.get(0); 
    }

来源

2017-03-03 19:35:53 jklee

获取主键，而不是在一对多的关系

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