Spring JPA：重用该属性存储另一个对象时，ManyToMany关系未存储在数据库中

如果必须保留对象的“复杂”结构，则@ManyToMany关联会出现问题。

我有一个使用Spring Boot（spring-boot-starter-parent 2.2.5.RELEASE）和JPA（spring boot-starter-data-jpa）的Java应用程序。作为数据库，我尝试使用MySQL和H2。效果是一样的。

摘要1

我在创建类别时使用的是List对象，而在创建项目对象时使用的是相同的List对象。

Category cat1 = new Category("Cat 1", personList, null);
categoryService.create(cat1);
Item item1 = new Item("Item 1", personList, cat1);
itemService.create(item1);

• 类别记录在category_managers表中具有分配为经理的三个人记录
• 物料记录在item_persons表中具有分配为人员的三个人员记录

摘要2

我在创建类别时使用List对象，在创建项目对象时重用category.getManager列表。

Category cat2 = new Category("Cat 2", personList, null);
categoryService.create(cat2);
Item item2 = new Item("Item 2", cat2.getManagers(), cat2);
itemService.create(item2);

• 该项目在item_persons表中分配了三个人记录作为人
• !!! 该类别在category_managers表中没有经理分配

摘要3

现在，与摘要2中相同的过程进行了额外的分配和更新：

Category cat3 = new Category("Cat 3", personList, null);
categoryService.create(cat3);
cat3.setManagers(personList);
categoryService.update(cat3);
Item item3 = new Item("Item 3", cat3.getManagers(), cat3);
itemService.create(item3);

• 该类别具有分配为经理的三个人记录（请参阅category_managers表）
• 该物料具有分配为人员的三个人员记录（请参阅item_persons表）。

问题

为什么摘要2中的作业没有保留？
我没记账吗
我会误会幕后发生的事情吗？

这可能是一个特殊的用例，但是我是否需要重新阅读一些主题或章节？

斯图加特的问候

详细说明

模型结构

Person.java：

@Entity
public class Person {
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String firstName;
    private String lastName;
    
    // Constructors -------------------------------------------------------------------------------
    
    public Person() {}
    
    public Person(String firstName, String lastName) {
        super();
        this.firstName = firstName;
        this.lastName = lastName;
    }
    
    // Getters / Setters --------------------------------------------------------------------------
    
    // ...
    
}

Category.java：

@Entity
public class Category {
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    @ManyToMany
    private List<Person> managers;
    @OneToMany(fetch = FetchType.LAZY)
    @JoinColumn(name="category_id", updatable = false)
    private List<Item> items;
    
    // Constructors -------------------------------------------------------------------------------
    
    public Category() {}
    
    public Category(String name, List<Person> managers, List<Item> items) {
        super();
        this.name = name;
        this.managers = managers;
        this.items = items;
    }
    
    // Getters / Setters --------------------------------------------------------------------------
    
    // ... 
}

Item.java：

@Entity
public class Item {
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    @ManyToMany
    private List<Person> persons;
    @ManyToOne
    private Category category;
    
    // Constructors -------------------------------------------------------------------------------
    
    public Item() {}
    
    public Item(String name, List<Person> persons, Category category) {
        super();
        this.name = name;
        this.persons = persons;
        this.category = category;
    }
    
    // Getters / Setters --------------------------------------------------------------------------
    
    // ...
}

财产人应包括分配给特定物品的人。属性类别应包含为多个项目分配的可选类别。类别的管理者将分配给其项目的人员属性。

非常简单直接。

相应的存储库接口和服务类

存储库接口扩展了JpaRepository接口。服务类包含用于创建，获取，更新和删除数据库中记录的方法。（（我只是发布其中之一））

@Service
public class PersonService {
    
    @Autowired
    PersonRepository personRepo;
    
    public Person create(Person person) {
        return personRepo.save(person);
    }
    public Person get(long id) {
        return personRepo.getOne(id);
    }
    public void update(Person person) {
        personRepo.save(person);
    }
    public void delete(long id) {
        personRepo.deleteById(id);
    }
    
}

三个测试案例

这里的代码演示了在数据库中创建记录时遇到的问题。有3种情况。

@Component
public class DatabaseInitializer implements CommandLineRunner {
    
    @Autowired
    CategoryService categoryService;
    @Autowired
    ItemService itemService;
    @Autowired
    PersonService personService;
    
    @Override
    public void run(String... args) throws Exception {
        
        // Create persons
        Person max = personService.create(new Person("Max", "Mad"));
        Person sally = personService.create(new Person("Sally", "Silly"));
        Person bob = personService.create(new Person("Bob", "Bad"));
        List<Person> personList = Arrays.asList(max, sally, bob);
        
        
        // Case 1 (this works)
        // Here we will use the personList object for both the category and the item.
        Category cat1 = new Category("Cat 1", personList, null);
        categoryService.create(cat1);
        Item item1 = new Item("Item 1", personList, cat1);
        itemService.create(item1);
        // => The category record has three person records assigned as managers (see. category_managers table)
        // => The item record has three person records assigned as persons (see. item_persons table)
        
        // Case 2 (this doesn't work)
        // Here we will use the personList object to create the category and reuse the category.getManager list to create the item.
        Category cat2 = new Category("Cat 2", personList, null);
        categoryService.create(cat2);
        Item item2 = new Item("Item 2", cat2.getManagers(), cat2);
        itemService.create(item2);
        // => The category has no managers assignments (see. category_managers table) WHY??
        // => The item has three person records assigned as persons (see. item_persons table)
        
        
        // Case 3 (workaround of case 2)
        // Here we will do the same as in case 2, but will do an extra assignment of the managers of the category
        Category cat3 = new Category("Cat 3", personList, null);
        categoryService.create(cat3);
        cat3.setManagers(personList);
        categoryService.update(cat3);
        Item item3 = new Item("Item 3", cat3.getManagers(), cat3);
        itemService.create(item3);
        // => The category has three person records assigned as managers (see. category_managers table)
        // => The item has three person records assigned as persons (see. item_persons table)
        
        
    }

}