建模单个子表/多个父表

我正在努力对共享单个表（子表）的多个表（父表）之间的关系进行建模。

给定以下父表1（其他父表2,3等类似）：

@Entity
@Table (name="parent1")
public class ParentEntity1 {

    @Id 
    @Column(name="id")
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Integer id;

    // relationship with child table

    @OneToMany(mappedBy = "parentId", fetch = FetchType.LAZY, cascade = CascadeType.ALL, orphanRemoval = true)
    private Set<ChildEntity> children;


    // other columns of parent 1


}

以及以下子表：

@Entity
@Table (name="children")
public class ChildEntity {

    @Id 
    @Column(name="id")
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Integer id;

    // points to the parent type; 1, 2, and so on
    @Column(name="parent_type")
    private Integer parentType;

    @Column(name="parent_id")
    private Integer parentId;

    // some other data that belongs to ChildEntity

}

在创建关系时如何保留一个父对象和多个子对象？请注意，父类型是子表中的一列，并且父 ID 和子 ID 都是自动生成的。

您应该做的是将 ParentEntity 实际附加到 ChildEntity。在 JPA 中，m:1 的这种关系是通过 @JoinColumn 实现的。您不需要将父类型存储在子实体中 - 这将是糟糕设计的标志。父级的类型应尽可能接近其实体。因此，只需将其放入该实体中即可：

@Entity
@Table (name="children")
public class ChildEntity {

    @Id Integer parentId;
    @Column(name="id")
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Integer id;

    @ManyToOne
    @JoinColumn(name="parent")
    private ParentEntity parent;

    // some other data that belongs to ChildEntity
    //all setters and getters / lombok

}

@MappedSuperclass
public abstract class ParentEntity {
    @OneToMany(mappedBy = "parentId", fetch = FetchType.LAZY, cascade = CascadeType.ALL, orphanRemoval = true)
    private Set<ChildEntity> children;    


    @Column(name="parent_type")
    private Integer parentType;

    //all setters and getters / lombok
}


@Entity
@Table (name="parent1")
public class ParentEntity1 extends ParentEntity {

    @Id 
    @Column(name="id")
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Integer id;    

    // other columns specific to parent 1

    //all setters and getters / lombok
}


@Entity
@Table (name="parent2")
public class ParentEntity2 extends ParentEntity {

    @Id 
    @Column(name="id")
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Integer id;    

    // other columns specific to parent 2

    //all setters and getters / lombok
}

为了让父母有几个孩子，你可以这样做：

ParentEntity pOne = new ParentEntity1();
ParentEntity pTwo = new ParentEntity2();

ChildEntity c1 = new ChildEntity();
ChildEntity c2 = new ChildEntity();
ChildEntity c3 = new ChildEntity();

Set<ChildEntity> childrenOne = new HashSet<>();
childrenOne.add(c1);
childrenOne.add(c2);

Set<ChildEntity> childrenTwo = new HashSet<>();
childrenTwo.add(c3);

pOne.setChildren(childrenOne);
pOne.setType(1);

// ... 
em.getTransaction().begin();
em.persist(pOne);
em.getTransaction().commit();
// ...

pTwo.setChildren(childrenTwo);
pTwo.setType(2);


// ... 
em.getTransaction().begin();
em.persist(pTwo);
em.getTransaction().commit();
// ... 

就这样。请注意，您永远不会实例化 ParentEntity 类（因为它是抽象的）。您可以有多个父表，其中每个父表通过 id 引用单个子表。此外，您可以通过继承 ParentEntity 将有关每个后续父实体类型的信息存储在其类中。我希望它能解决你的问题。