Question

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？为什么我在单表策略中需要fk？
为什么要创建主键（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！

员工：

@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(){}
}

管理器：

@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(){}
}

Answer 1

由于您已选择SINGLE_TABLE作为继承策略，因此Employee和Manager都存储在同一个表中。

默认情况下，单向一对多关系（在您的情况下为Employee.manager）由JPA在附加联接表的帮助下表示（就像多对多关系一样）。在您的情况下，它恰好加入employees表自己。如果您想在源实体表中使用外键，则需要使用@JoinColumn

这与第1点有关。你能用下面的代码解释一下你想要完成的事吗？ Manager已经有一个id（来自其父实体）

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

如果您切换到使用@JoinTable的事件，employees表格中仍会有外键（您认为Employee - Manager关系怎么样？在数据库中表示？）

由于以下代码：

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

好问题。我认为这是因为Department是一个Manager，当Employee.manager实际指向Department时，JPA需要这种方式。为什么要让Department继承Manager？ Department有一个Manager（@OneToOne/@ManyToOne）更合乎逻辑吗？这会简化事情，你知道
https://en.wikibooks.org/wiki/Java_Persistence/OneToMany#Example_of_a_OneToMany_using_a_JoinTable_database https://en.wikibooks.org/wiki/Java_Persistence/Inheritance（这些确实是Google的最佳结果......）

修改

要实施Department has-a Manager关系，您需要进行以下更改：

    @Entity
    public class Department {
        @Id
        private int id;

        @ManyToOne(mappedBy = "department")
        private Manager manager;
        ...
    }

Department.manager并非绝对必要，但我已将其包含在内，以防您想要双向关系（我没有注意到Manager.department字段已经在{ {1}}我第一次读你的代码时）

Answer 2

不确定你想要实现的目标。但肯定你的代码看起来有点奇怪。

1）表employees_employees已经创建，因为您已经提到管理器和部门之间的映射为一对多。一个表策略默认为一对一映射。

2）问题是你有2个同名id的变量;一个在employee表中，另一个在manager表中。

3）是正确的行为。您在员工类中使用了@id注释。因此创建了主键。

4）因为你使用了单表策略。部门的超级父级是员工，因此部门的主要关键栏与员工的主要关键栏相同。所以department_id引用了employee表的id。

5）告诉我们您想要达到的目标，我们可以为您提供帮助。或者浏览一下hibernate教程。

JPA中的继承策略

2 个答案: