JPA级联仍然存在-多对一 [英] JPA cascade persist - many to one

问题描述

我有一对多的关系，并且我试图保留一个子实体.

I have a many to one relationship and I am trying to persist a child entity.

public class Office
{
public int id;
public int grades;
@OneToMany
public set<Employee> employees;
}

public class Employee{
@GeneratedValue(strategy=GeneratedValue.identity)
public int empid;
@ManyToOne(cascade=cascadeType.ALL)
public Office office;
}

Office ID已存在于数据库中.但是员工不是. 现在，如果我要添加一名员工，并且他的成绩必须进入办公室数据库.

Office Id is already present in the database. But employee is not. Now if i am adding an employee and his grades must go into the office database.

当我执行以下操作时，成绩不会保存

When i do the following operation,grades are not getting saved

Office office = new Office();
office.setId(23);
office.setGrades(5);
employee.setOffice(office);
em.persist(employee);

如何通过一次操作将成绩保存到办公室表中

How to save grades into the office table in a single operation

推荐答案

首先，修复您的映射.

First, fix your mapping.

关联是双向的，并且必须使用mappingBy属性将一侧(另一侧)标记为另一侧的反向:

The association is bidirectional, and one of the side (the one side) must be marked as the inverse of the other using the mappedBy attribute:

@OneToMany(mappedBy = "office")
public set<Employee> employees;

员工只是办公室的员工之一.删除单个员工时，您真的要删除整个办公室吗?如果不是，为什么将cascade=cascadeType.ALL放在@ManyToOne上?这些注释会产生后果.不要在不了解它们的情况下使用它们.

The employee is only one of the employees of the office. Do you really want to delete the entire office when you delete a single employee? If not, why do you put a cascade=cascadeType.ALL on the @ManyToOne? Those annotations have consequences. Don't use them without understanding them.

现在可以真正回答问题了.如果数据库中已经存在该办公室，则不应建立一个新的办公室.从数据库中获取并更新它:

Now to really answer the question. If the office already exists in the database, you should not build a new one. Go fetch it from the database and update it:

Office office = em.find(Office.class, 23);
// office is now attached, and any change you make on the entity will be written to the database
office.setGrade(5);

现在，您还可以将办公室附加到新员工上.但是，由于这是双向关系，因此您还应该初始化关联的另一端，以保持对象图的连贯性:

Now you may also attach the office to the new employee. But since it's a bidirectional relationship, you should also initialize the other side of the association to keep the object graph coherent:

employee.setOffice(office);
office.addEmployee(employee);
em.persist(employee);

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