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

问题描述

也许这是一个愚蠢的问题，但它困扰着我.

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

我有一个员工和车辆的双向一对多关系.当我第一次将 Employee 持久化到数据库中时(即它没有分配的 ID)，我也希望它的关联车辆被持久化.

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用，除了我保存的 Vehicle 实体没有自动获取关联的 Employee 映射，并且在数据库中，Vehicle 表中的 employee_id 外键列为空.

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，是否有可能在保留 Employee 本身的同时保留 Vehicle 的雇员?我意识到需要先保存 Employee ，然后再保存 Vehicle .JPA 可以为我自动执行此操作吗?或者我是否必须执行以下操作:

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！

根据要求，这是我的映射(不包括所有其他方法等)

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;

    ...
}

我刚刚意识到我应该在我的 Employee 类中定义以下方法:

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);
}

现在上面的代码看起来像这样:

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、更干净.感谢大家的帮助！

Much simpler and cleaner. Thanks for your help everyone!

推荐答案

在持久化 Employee 之前，您必须在 Vehicle 上设置相关的Employee.

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);

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