JPA - 坚持一对多的关系 [英] JPA - Persisting a One to Many relationship

问题描述

也许这是一个愚蠢的问题，但它让我烦恼。

Maybe this is a stupid question but it's bugging me.

我有一对双向的员工与车辆的关系。当我第一次在数据库中保留一个Employee（即它没有分配的ID）时，我也希望它的相关Vehicle保持不变。

I have a bi-directional one to many relationship of Employee to Vehicles. When I persist an Employee in the database for the first time (i.e. it has no assigned ID) I also want its associated Vehicles to be persisted.

目前这对我来说很好，除了我保存的Vehicle实体没有自动映射关联的Employee，而在数据库中的employee_id外键列车辆表是空的。

This works fine for me at the moment, except that my saved Vehicle entity is not getting the associated Employee mapped automatically, and in the database the employee_id foreign key column in the Vehicle table is null.

我的问题是，是否可以在持有员工本身的同时保持车辆的员工？我意识到需要首先保存员工，然后保存车辆。 JPA可以自动为我做这个吗？或者我必须做以下事情：

My question is, is it possible to have the Vehicle's employee persisted at the same time the Employee itself is being persisted? I realise that the Employee would need to be saved first, then the Vehicle saved afterwards. Can JPA do this automatically for me? Or do I have to do something like the following:

Vehicle vehicle1 = new Vehicle();
Set<Vehicle> vehicles = new HashSet<Vehicle>();
vehicles.add(vehicle1);

Employee newEmployee = new Employee("matt");
newEmployee.setVehicles(vehicles);
Employee savedEmployee = employeeDao.persistOrMerge(newEmployee);

vehicle1.setAssociatedEmployee(savedEmployee);
vehicleDao.persistOrMerge(vehicle1);

谢谢！

编辑：按要求，这是我的映射（没有所有其他方法等。）

As requested, here's my mappings (without all the other methods etc.)

@Entity
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name="employee_id")
    private Long id;

    @OneToMany(mappedBy="associatedEmployee", cascade=CascadeType.ALL)
    private Set<Vehicle> vehicles;

    ...

}

@Entity 
public class Vehicle {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name="vehicle_id")
    private Long id;

    @ManyToOne
    @JoinColumn(name="employee_id")
    private Employee associatedEmployee;

    ...
}

我刚才意识到我应该在我的Employee类上定义了以下方法：

I just realised I should have had the following method defined on my Employee class:

public void addVehicle(Vehicle vehicle) {
    vehicle.setAssociatedEmployee(this);
    vehicles.add(vehicle);
}

现在上面的代码如下所示：

Now the code above will look like this:

Vehicle vehicle1 = new Vehicle();

Employee newEmployee = new Employee("matt");
newEmployee.addVehicle(vehicle1);
Employee savedEmployee = employeeDao.persistOrMerge(newEmployee);

更简单，更清洁。感谢大家的帮助！

Much simpler and cleaner. Thanks for your help everyone!

推荐答案

在持久化员工之前，您必须在Vehicle上设置associatedEmployee。

You have to set the associatedEmployee on the Vehicle before persisting the Employee.

Employee newEmployee = new Employee("matt");
vehicle1.setAssociatedEmployee(newEmployee);
vehicles.add(vehicle1);

newEmployee.setVehicles(vehicles);

Employee savedEmployee = employeeDao.persistOrMerge(newEmployee);

这篇关于JPA - 坚持一对多的关系的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！