如何使用与基本类型的多个键的值具有相同实体的 Map

Posted 2023-04-12

【中文标题】如何使用与基本类型的多个键的值具有相同实体的 Map

【英文标题】：How to use a Map that has the same entity as the value for multiple keys of a basic type

【发布时间】：2020-07-10 18:14:50

【问题描述】：

我正在阅读 Pro JPA 2，它给出了以下示例：

@Entity
public class Department 
    @Id private int id;

    @OneToMany(mappedBy="department")
    @MapKeyColumn(name="CUB_ID")
    private Map<String, Employee> employeesByCubicle;
    // ...

书中提到CUB_ID 将是Employee 实体表上的一列。但是，如何支持（或可以更改为支持）在多个隔间工作的同一员工，例如地图包含

"cube A1": "id":"1","name":"John Doe",
 "cube A2": "id":"1","name":"John Doe"

【参考方案1】：

您可以通过将部门端的关系更新为@ManytoMany 和@JoinTable 来做到这一点（您可以将@ManyToOne 保留在员工端或单向）

    @ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
    @JoinTable
    @MapKeyColumn(name="CUB_ID", nullable = true)
    private Map<String, Employee> employeesByCubicle;

输出：如您所见，它在两个隔间中都保留了对同一员工的引用

        Employee employee = new Employee(1l, "Kav");
        Map<String, Employee> map = new HashMap<>();
        map.put("Cube1", employee);
        map.put("Cube2", employee);
        Department dep = new Department(1l, map);
        employee.setDepartment(dep);
        //Saved a department and an employee with 2 cubicles
        departmentRepository.save(dep);

        Department retrieved = departmentRepository.findById(employee.getId())
                                                    .get();
        System.out.println(retrieved);
        //Output
        //Departmentid=1, employeesByCubicle=Cube1=Employeeid=1, name='Kav',
        //                                     Cube2=Employeeid=1, name='Kav'

您可以找到运行、插入数据、获取、打印然后关闭的程序。 https://github.com/kavi-kanap/stack-overflow-62840098