mysql - 使用 JPA 将连接表映射到 Java 中的 Map 对象

Question

我有五个 MySQL 表

• employee
• qualification
• institute qualification_institute (id, qualification_id,institute_id)
• employee_qualification_institute (employee_id, qualification_institute_id)

qualification_institute employee_qualification_institute 定义了可获得的资格和机构之间的多对多连接告诉我们特定员工在哪个机构获得了他/她的资格。 EclipseLink 生成一个 Set<QualificationInstitute> Employee 中的字段类，但是我想要Map<Qualification, Institute>而不是它。我应该如何注释该 map ？由于我具有基于属性的访问权限，我是否需要在其 getter 方法中实现查询？

Qualification类已经包含一个 Set<Institute>场而类Institute持有Set<Qualification> .如果我可以在 Employee 中注释 map ，也许我可以避免必须有一个明确的 EmployeeQualificationInstitute类。

Answer 1

在这种情况下你不能。由于按照设计，每个学院的资格证书并不是唯一的。我引用了 Map 的 JavaDoc:

An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.

然而，EmployeeQualificationInstitute 类不是必需的。

例子:

@Entity
public class Employee
{
    @Id
    @GeneratedValue
    private Long id;
    @JoinTable(name="EmployeeQualificationInstitute",
        joinColumns={
            @JoinColumn(name = "id", referencedColumnName = "employee_id")
        }
    )
    private Set<QualificationInstitute>
}

@Entity
public class Qualification
{
    @Id
    @GeneratedValue
    private Long id;
}

@Entity
public class Institute
{
    @Id
    @GeneratedValue
    private Long id;
}

@Entity
public class QualificationInstitute
{
    @EmbeddedId
    private QualificationInstitutePK key;

    @MapsId(value = "qualification_id")
    @ManyToOne
    @JoinColumn(name = "qualification_id", referencedColumnName = "id")
    private Qualification qualification;

    @MapsId(value = "institute_id")
    @ManyToOne
    @JoinColumn(name = "institute_id", referencedColumnName = "id")
    private Institute institute;
}

@Embeddable
public class QualificationInstitutePK
{
    @Column(name = "qualification_id")
    private Long qualification_id;

    @Column(name = "institute_id")
    private Long institute_id;
}