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3. JPA对象关系映射 -- 级联操作

对象映射关系分为单向关系和双向关系，单向关系只在一方对象上存在对方对象，双向关系是在双方对象上存在彼此对象。

一、单向关系

public class Department implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer departmentId;

    private String departmentCode;
    
    @OneToMany(cascade = {CascadeType.ALL})
    @JoinColumn
    private List<Employee> employeeList;
}

在没有 @JoinColumn 时，将多增加一个中间关系表，由此表来维护两个对象关系，增加之后只有两个表，由 Employee 表维护关系。默认情况下jpa会使用主键来做关联，并在子表中增加外键约束。

二、单向关系使用code关联

在设计表结构时，主键一般会使用自增ID，但在做子表关联时由于分布式结构原因不想使用自增ID来做关系维护，则可自定义字段 code 来维护关系，如下：

public class Department implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer departmentId;

    private String departmentCode;

    @OneToMany(cascade = {CascadeType.ALL})
    @JoinColumn(name = "departmentCode", referencedColumnName = "departmentCode")
    private List<Employee> employeeList;
}

在employee表中会增加一个 department_code 字段来维护关联关系。

注意：从add操作中的SQL可以看出，employee是先insert之后，再去update关系字段的，多一步update。

Hibernate: insert into department (department_code, department_name) values (?, ?)
Hibernate: insert into employee (employee_code, employee_name) values (?, ?)
Hibernate: insert into employee (employee_code, employee_name) values (?, ?)
Hibernate: update employee set department_code=? where employee_id=?
Hibernate: update employee set department_code=? where employee_id=?

三、双向关系

public class Department implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer departmentId;

    private String departmentCode;

    @OneToMany(cascade = {CascadeType.ALL}, mappedBy = "department")
    private List<Employee> employeeList;
}

public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer employeeId;

    private String employeeCode;

    @ManyToOne
    private Department department;
}

使用 mappedBy 将去掉中间关系表，由employee维护department的关系。

注意事项：

1.在add操作中子对象employee必须设置主对象，否则数据库层面是没有维护外键关系的，如：

@Test
public void add(){
    Department department = Department.builder()
            .departmentCode("D001")
            .departmentName("部门1")
            .build();

    Employee employee = Employee.builder()
            .employeeCode("E001")
            .employeeName("员工1")
            .build();
    // 如无此操作，将无关联关系        
    employee.setDepartment(department);
    
    List<Employee> employeeList = new ArrayList<>();
    employeeList.add(employee);

    department.setEmployeeList(employeeList);
    departmentRepository.save(department);
}

2.此种设置是不需要额外update关系的

Hibernate: insert into department (department_code, department_name) values (?, ?)
Hibernate: insert into employee (department_department_id, employee_code, employee_name) values (?, ?, ?)
Hibernate: insert into employee (department_department_id, employee_code, employee_name) values (?, ?, ?)

3.关闭子对象中的父对象toString，避免无限循环调用