java - jpa Embeddables 与实体的双向关系

Question

嵌入式和实体之间是否可能存在双向关系或仅存在单向关系？

@Entity
public class Employee {
  @Id
  private long id;
  ...
  @Embedded
  private EmploymentPeriod period;
  ...
}
@Embeddable 
public class EmploymentPeriod {
  @Column(name="START_DATE")
  private java.sql.Date startDate;

  @Column(name="END_DATE")
  private java.sql.Date endDate;

  @OneToMany
  private EntityABCD entityABCD ;
  ....
}
@Entity
public class EntityABCD {
  @Id
  private long id;
  ...
  @ManyToOne(mappedby="entityABCD")
  private EmploymentPeriod period;
  ...
}

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根据 JPA 规范:2.5 可嵌入类

 An entity cannot have a unidirectional relationship to the embeddable class of another entity (or itself).

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请按照 JPA 规范的规定澄清上述行。

Answer 1

当每个实体都指向另一个实体时，关系是双向的。
如果只有一个实体指向另一个实体，则关系是单向。

在您的具体情况下，句子:

An entity cannot have a unidirectional relationship to the embeddable class of another entity (or itself)

可以翻译为:

EntityABCD entity cannot have a unidirectional relationship to the embeddable class EmploymentPeriod of another Employee entity.

换句话说:

If Employee entity has an embedded EmploymentPeriod, it is not possible to define an unidirectional relationship from EntityABCD entity to embeddable EmploymentPeriod of entity Employee.

为什么？

由于可嵌入对象没有自己的标识(缺少主键)，因此只需将其视为封装它的实体的一部分。从数据库的角度来看，嵌入对象与其余实体属性存储在一行中。

由于上述原因，如果有人尝试从 EntityABCD 创建单向关系，则 EmploymentPeriod由于缺乏 EmploymentPeriod 的身份，这是根本不可能的，因此不可能在嵌入对象中创建外键。

如何解决外键问题？

外键需要在可嵌入类之外物理创建，这取决于关系类型。一个例子(:

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

  @Embedded
  private EmploymentPeriod period;
}

@Embeddable
public class EmploymentPeriod {
    @ManyToOne //owning relationship
    @JoinColumn //FK in EMPLOYEE table (by default: ENTITYABCD_ID) 
    private EntityABCD entityABCD;

    @ManyToMany //owning relationship
    @MapKey(name="id") //refers to EntityABCD.id
    @JoinTable //FK in the join table (by default: EMPLOYEE_ENTITYABCD)
    private Map<Long, EntityABCD> entitiesABCD;
}

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

    @OneToMany(mappedBy = "period.entityABCD") //non-owning relationship
    List<Employee> employee;

    @ManyToMany(mappedBy="period.entitiesABCD") //non-owning relationship
    private List<Employee> employees;
}

当嵌入对象内存在双向关系时，它们被视为存在于所属实体 (Employee) 中目标实体 (EntityABCD) 指向所属实体，而不是嵌入对象 (EmploymentPeriod)。

值得一提的是，embeddables 只能嵌入:

• 其他嵌入式
• 与实体的关系
• 基本/可嵌入类型的元素集合