地图连接表的Map对象在Java中使用JPA

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定义了可获得的资格和机构之间的ManyToMany连接，而employee_qualification_institute告诉我们哪个机构的特定员工获得了其资格。 EclipseLink在Employee类中生成一个Set<QualificationInstitute>字段，但是我想要一个Map<Qualification, Institute>而不是它。我应该如何注释该地图？由于我有基于属性的访问，我需要在其getter方法中实现一个查询吗？

Qualification类已包含Set<Institute>字段，而Institute类包含Set<Qualification>。如果我可以在Employee内注释一张地图，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。由于通过设计，每个研究所的资格并非唯一。我引用了Map的JavaDoc：

将键映射到值的对象。地图不能包含重复的键;每个键可以映射到最多一个值。

然而EmployeeQualificationInstitute类没有必要。

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; 
}