java - Hibernate继承，父类应该根据决策者是子类还是父类

Question

尝试在Hibernate中实现继承。

以下是架构

这是什么，类是，

//Grand Parent Class
@Entity
@Table(name="grand_parent")
public class GrandParent{//consider @id}

//Parent Class
@Entity
@Table(name = "parent") 
@Inheritance(strategy = InheritanceType.JOINED)
@DiscriminatorColumn(name = "decider", discriminatorType = DiscriminatorType.STRING)
public class Parent{//consider @id}

//ChildX class
@Entity
@Table(name = "childX")
@PrimaryKeyJoinColumn(name="id")
@DiscriminatorValue("X")
public class ChildX() extends Parent{//consider value}

//ChildY class
@Entity
@Table(name = "childY")
@PrimaryKeyJoinColumn(name="id")
@DiscriminatorValue("Y")
public class ChildY extends Parent(//consider value){}

//ChildZ class
@Entity
@Table(name = "childZ")
@PrimaryKeyJoinColumn(name="id")
@DiscriminatorValue("Z")
public class ChildZ() extends Parent{//consider value}

用例:

• 如果决策者为“K”，并且需要保存 4 条记录，则应添加 4 条父记录
• 如果决策者为“X/Y/Z”，需要保存 4 条记录，则应添加 1 条父记录和 4 条 ChildX/ChildY/ChildZ 记录。

但是，当决策者为“K”时，父表应被视为单个子表，并且当决策者为“X/Y/Z”时，它必须充当父表

但在上面的类图中，每当决策者为“X/Y/Z”时，ChildX/ChildY/ChildZ 中保存了 4 条记录，而父表中没有记录。

还有如何检索上述记录。

编辑

@Entity
@Table(name="grand_parent")
public class GrandParent{
    @OneToMany(mappedBy = "parentRecord", 
        fetch = FetchType.LAZY, 
        cascade = CascadeType.ALL)
@Cascade(org.hibernate.annotations.CascadeType.DELETE)
private List<parent> parentList;
}

//Parent Class
@Entity
@Table(name = "parent") 
public class Parent{
 @ManyToOne()
@JoinColumn(name = "fk_gp_id")
private GrandParent parentRecord;

 @OneToMany(mappedBy = "childrecord", 
        fetch = FetchType.LAZY, 
        cascade = CascadeType.ALL)
@Cascade(org.hibernate.annotations.CascadeType.DELETE)
private List<Child> childList;
}

@Entity
@DiscriminatorColumn(name = "decider", discriminatorType = DiscriminatorType.STRING)
public abstract class Child(){
@ManyToOne(optional = false)
@JoinColumn(name = "fk_parent_id")
private parent childrecord;
}

//ChildX class
@Entity
@Table(name = "childX")
@DiscriminatorValue("X")
public class ChildX() extends Parent{//consider value}
......

添加..

GrandParent gp = new GrandParent();
Parent p = new Parent();
ChildX ch = new ChildX();
ch.setChildrecord(p);
p.setChildList(//Array added ch);
p.setParentRecord(gp);
gp.setParentList(//Array added p);
persist(gp);

现在我收到错误:

Application error : com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Table 'Child' doesn't exist

Answer 1

当您使用(联合)继承时，当您保留子项时，您始终会在父表和子表中插入一行。这意味着每个 parent 最多有一个 child 。在您的表图中，您有多个具有相同父级的子级，这不是继承，这是一个 OneToMany 实体关系，这由您的 ER 图确认。

我的猜测是您正在寻找这样的东西:

@Entity
public class Parent {
    @Id
    private long id;

    @OneToMany(mappedBy = "parent")
    private Collection<Child> children; 
}


@Entity
@DiscriminatorColumn(name = "decider", discriminatorType = DiscriminatorType.STRING)
public abstract class Child {
    @Id
    private long id;

    @ManyToOne
    private Parent parent;
    // other common fields
}

@Entity
@DiscriminatorValue("X")
public class ChildX extends Child {
    // specific fields for child x
} 

// more children types

一个父级可以有多个子级，并且这些子级可以是不同的类型。子基类具有引用父级的外键列，根据我的经验，继承基类通常最终是抽象的。

编辑:以下是存储父级和特定子级的代码示例。

EntityManager em = emf.createEntityManager();
try {
    em.getTransaction().begin();
    Parent p = new Parent();
    ChildX childX = new ChildX();
    childX.setParent(p);
    em.persist(p);
    em.persist(childX);
    em.getTransaction().commit();
} finally {
    em.close();
}