獲取主鍵，而不是在一對多的關係

整個對象，我有以下類別：獲取主鍵，而不是在一對多的關係

@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 

}

我創建了一個Web服務的球衣與CRUD操作的DAO類治療數據。通過這個實現，當我調用GET方法時，我得到之內的整個Elements對象。這就是所謂的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; 
}

我不希望這fetch = FetchType.EAGER提取類型，因爲我只需要的Elements的PK，但是經過一番研究，這是我發現讓我的服務工作的唯一辦法。我試過this和this的方法，但我一直無法使它工作。

來源

2017-03-03 Juan

您可以在查詢中使用左外連接提取。

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

現在你可以保持fetch = FetchType.LAZY（這是默認值）。

來源

2017-03-03 11:00:56

它沒有工作，返回的結果相同 – Juan

前取不急於類似，它只是另一種方式來獲得渴望 –

是的數據，但在這種情況下，你只有當你需要得到相關的對象。不是每次。 –

嘗試將@XmlTransient註釋添加到您要忽略的Elements類中的字段。這樣的：

@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 
}

來源

2017-03-03 12:01:25

您可以使用多選與Tuple，只能選擇特定的列。

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); 
    }

來源

2017-03-03 19:35:53 jklee

獲取主鍵，而不是在一對多的關係

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