Question

我有以下课程：

@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

}

我创建了一个带有CRUD操作的平针织webservice和DAO类来处理Alarms数据。通过此实现，当我调用GET方法时，我会在Elements个内部获得整个Alarms对象。这是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;
}

我不想要这个fetch = FetchType.EAGER获取类型，因为我只需要Elements的PK，但经过一些研究后，这是我发现使我的服务工作的唯一方法。我尝试了this和this方法，但我无法使其发挥作用。

Answer 1

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）。

Answer 2

尝试将@XmlTransient注释添加到要忽略的Elements类中的字段。像这样：

@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
}

Answer 3

您可以将多选与Tuple一起使用，仅用于选择特定列。

    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);
    }

在OneToMany关系中获取主键而不是整个对象

3 个答案: