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在我的示例中，我有两个实体要持久化。但是当我尝试使用 @MapKeyColumn 注释时，我对如何使用这个注释以及如何持久化数据感到困惑。这是“PRO JPA2 book”中解释的示例。当我尝试运行此代码时，它将生成异常。

两个实体：

@Entity
@Access(AccessType.FIELD)
public class Employee {
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private int id;
private String name; 
private String salary;

@Enumerated(EnumType.STRING)
private EmployeeType type;

@ManyToOne
@JoinColumn(name="dept_id")
private Department deparment;

和

@Entity
public class Department {
@Id 
@Column(name="dept_id")
@GeneratedValue(strategy=GenerationType.AUTO)
private int id;
private String Name;

@OneToMany(mappedBy="deparment")
@MapKeyColumn(name="cabin_id")
private Map<String , Employee> empCabin;
    ......................................
   public Map<String, Employee> getEmpCabin() {
    return empCabin;
}

public void assignEmpToCabin(Employee emp , String cabinNo) {
    System.out.println(emp+"   "+cabinNo);
    getEmpCabin().put(cabinNo, emp);

    if(emp.getDeparment() != null){
        emp.getDeparment().removeEmpFromDept(emp);
    }

    emp.setDeparment(this);
}

public void removeEmpFromDept(Employee emp) {
    Iterator<Map.Entry<String, Employee>> entrySet = getEmpCabin().entrySet().iterator();
    if(entrySet.hasNext()){
        Employee current = ((Map.Entry<String, Employee>) entrySet.next()).getValue();
        if(current.getId() == emp.getId()){
            System.out.println("dept remove");
            entrySet.remove();
            current.setDeparment(null);
        }
    }
}

我的服务类功能

public class EmployeeService {
private EntityManager em;
.............
public void setEmpCabin(Employee emp , Department dept , String cab_id) {
    dept.assignEmpToCabin(emp, cab_id);
}

我的主要代码：

tr.begin();
        Department dept = deps.createDepartment("Sci");
        Employee emp = es.createEmployee("Harmeet Singh", "500" , "5684", dept);
        es.setEmpCabin(emp, dept, "10");
tr.commit();

当我尝试使用 setEmpCabin(...) 函数插入数据时，它会生成异常

Caused by: org.hibernate.exception.GenericJDBCException: Field 'cabin_id' doesn't have a default value
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