JPA中的继承策略

我刚接触JPA，所以初学者的问题：

• 为什么Hibernate Provider会使用employees_employees创建第二个表？我的示例中的代码是创建第二个表的原因？

• 为什么manager_id列成为员工的id列的fk？为什么我需要单表策略中的fk？

• 为什么创建主键（屏幕截图中的金色）？

• 在第二个表 employees_employees 中创建了department_id作为员工表中的fk到id，为什么？

• 来自互联网的任何资源，以便了解这一点，将是HelpFul！

雇员：

@Entity
@Table (name = "employees")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "test" , discriminatorType = DiscriminatorType.STRING)
public class Employee {

public Employee(int id, String name, String lastname, double salary, String spec) {
    this.id = id;
    this.name = name;
    this.salary = salary;
    this.specialization = spec;
    this.lastname = lastname;
}

public int getId() {
    return id;
}

public String getName() {
    return name;
}

public double getSalary() {
    return salary;
}

public void setId(int id) {
    this.id = id;
}

public void setName(String name) {
    this.name = name;
}

public void setSalary(double salary) {
    this.salary = salary;
}

@Override
public String toString() {
    return "Employee{" +
            "id=" + id +
            ", name='" + name + '\'' +
            ", salary=" + salary +
            ", spec='" + specialization + '\'' +
            '}';
}

public String getLastname() {
    return lastname;
}

public void setLastname(String lastname) {
    this.lastname = lastname;
}

@Id
@Column(name = "id")
private int id;

@Column(name = "name")
private String name;

@Column(name = "lastname")
private String lastname;

@Column(name = "salary")
private double salary;

@Column (name = "specialization")
private String specialization;

public Manager getManager() {
    return manager;
}

public void setManager(Manager manager) {
    this.manager = manager;
}

@OneToOne(cascade = {CascadeType.PERSIST,CascadeType.MERGE,CascadeType.REMOVE}, fetch = FetchType.EAGER)
private Manager manager;

public Employee(){}
}

经理：

@Entity
@DiscriminatorValue("M")
public class Manager extends Employee{

public Manager(){}


public Set<Departments> getDepartment() {
    return department;
}

public void setDepartment(Set<Departments> department) {
    this.department = department;
}

public String getManager_country() {
    return manager_country;
}

public void setManager_country(String manager_country) {
    this.manager_country = manager_country;
}

public String getManager_address() {
    return manager_address;
}

public void setManager_address(String manager_address) {
    this.manager_address = manager_address;
}

public String getManager_experience() {
    return manager_experience;
}

public void setManager_experience(String manager_experience) {
    this.manager_experience = manager_experience;
}

@Override
public String toString() {
    return "Manager{" +
            "department=" + department +
            ", manager_address='" + manager_address + '\'' +
            ", manager_experience='" + manager_experience + '\'' +
            ", manager_country='" + manager_country + '\'' +
            ", id=" + id +
            '}';
}

@OneToMany(fetch = FetchType.EAGER , cascade = {CascadeType.PERSIST,CascadeType.MERGE,CascadeType.REMOVE} )
private Set<Departments> department;

//private Employee employee;

public Manager(String manager_experience,String manager_address, String manager_country) {

    this.manager_experience = manager_experience;
    this.manager_address = manager_address;
    this.manager_country = manager_country;
}

@Column(name = "experience")
private String manager_experience;

@Column(name = "address")
private String manager_address;

@Column(name = "country")
private String manager_country;

@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = "manager_id" ,insertable = false , updatable = false)
private int id;
}

部门：

@Entity
@DiscriminatorValue("D")
public class Departments extends Manager{

public String getSales() {
    return sales;
}

public void setSales(String sales) {
    this.sales = sales;
}

public String getCostumer_support() {
    return costumer_support;
}

public void setCostumer_support(String costumer_support) {
    this.costumer_support = costumer_support;
}

public String getDevelopers() {
    return developers;
}

public void setDevelopers(String developers) {
    this.developers = developers;
}

public String getSystem_administrators() {
    return system_administrators;
}

public void setSystem_administrators(String system_administrators) {
    this.system_administrators = system_administrators;
}

public String getAccounting_department() {
    return accounting_department;
}

public void setAccounting_department(String accounting_department) {
    this.accounting_department = accounting_department;
}

//private Manager manager;

@Column(name = "costumer_support_department" , unique = false, updatable = true , insertable = true , nullable = true)
private String costumer_support;

@Column(name = "developers_department" , unique = false, updatable = true , insertable = true , nullable = true)
private String developers;

@Column(name = "sys_admins_department" , unique = false, updatable = true , insertable = true , nullable = true)
private String system_administrators;

@Column(name = "accounting_department" , unique = false, updatable = true , insertable = true , nullable = true)
private String accounting_department;

@Column(name = "sales_department" , unique = false, updatable = true , insertable = true , nullable = true)
private String sales;

public Departments(){}
}