java - 无法从数据库中获取多对多关系中的对象集合

Question

我在 Hibernate 中有以下层次结构:

@Entity
@Table(name = "elements")
@Inheritance(strategy=InheritanceType.JOINED)

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;

    @ManyToMany(cascade = CascadeType.PERSIST, mappedBy = "elementsCollection")
    private Collection<ElementsGroups> elementsGroupsCollection;

    .
    .
    More attributes, constructor and getter/setter
    .
    .
}

@Entity
@Table(name = "valves")
@PrimaryKeyJoinColumn(name="idelement")
@XmlRootElement
public class Valves extends Elements {

    @Basic(optional = false)
    @Column(name = "position")
    private int position;

    @Basic(optional = false)
    @Column(name = "status")
    private boolean status;

    .
    .
    More attributes, constructor and getter/setter
    .
    .
}

@Entity
@Table(name = "elementsgroups")
@XmlRootElement
public class ElementsGroups implements Serializable {

    private static final long serialVersionUID = 1L;

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

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

    @ManyToMany(cascade = {CascadeType.PERSIST})
    @JoinTable(name="joinelementsgroups", joinColumns={@JoinColumn(name="idgroup")}, inverseJoinColumns={@JoinColumn(name="idelement")})
    private Collection<Elements> elementsCollection;

    .
    .
    More attributes, constructor and getter/setter
    .
    .
}

这个想法是有一个“元素”父类(super class)来包装系统的一堆元素，这些元素共享一些特性和功能。这些元素可以分为几组。这是数据库结构:

当我持久化 Valve 子类的对象并将其添加到一组时，我可以看到数据库中的更改。像这样:

    SessionFactory sf = new Configuration().configure().buildSessionFactory();

    Session s = sf.openSession();
    Transaction tx = s.beginTransaction();  
    Valve v1 = new Valve();
    Valve v2 = new Valve();
    s.save(v1);
    s.save(v2);
    tx.commit();

    tx = s.beginTransaction();
    LinkedList<Elements> valves = new LinkedList<>();
    valves.add(v1);
    valves.add(v2);
    ge.setElementsCollection(valves);

    s.save(ge);
    tx.commit();

//Database: OK

当我获得 Valve 对象时，我可以通过它的 getter 方法获得该对象所属的组。当我尝试获取 ElementsGroups 对象时出现问题，我可以在其中获取它的所有属性，但是 elementsCollection 是空的而不是包含 Valve之前添加:

    tx = s.beginTransaction();
    Valve v1 = (Valve)s.get(Valve.class, 3);
    for(ElementsGroups g : v1.getElementsGroupsCollection())
        System.out.println("Valve belongs to: " + g);

    ElementsGroups ge = (ElementsGroups)s.get(ElementsGroups.class, 1);
    System.out.println("Group number of elements: " + ge.getElementsCollection().size());

    tx.commit();

输出:

Valve belongs to: model.ElementsGroups[ idgroup=1 ]
Group number of elements: 0

我已经实现了类似的关系，但没有显示这个问题，所以我猜这个问题可能与类的层次结构有关，但我是 Hibernate 的新手，不知道我是否缺少一些东西。

ps:为了方便阅读，我省略了部分代码，如有必要我可以添加

Answer 1

我想你忘了用 ElementGroups 实体填充 Valve 实体，这样双方就可以相互引用:

    tx = s.beginTransaction();
    LinkedList<Elements> valves = new LinkedList<>();

    Valve managedV1 = s.merge(v1);
    Valve managedV2 = s.merge(v2);

    valves.add(managedV1);
    valves.add(managedV2);

    managedV1.getElementsGroupsCollection().add(ge);
    managedV2.getElementsGroupsCollection().add(ge);

    ge.setElementsCollection(valves);

    s.save(ge);
    tx.commit();

此外，我认为您应该在第二笔交易中首先合并 Valves。