Question

我有两个实体类A和B，如下所示。

public class A{

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;    

    @OneToMany(mappedBy = "a", fetch = FetchType.LAZY, cascade = {CascadeType.ALL})
    private List<B> blist = new ArrayList<B>();

    //Other class members;

}

B组：

public class B{

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @ManyToOne
    private A a;

    //Other class members;

}

我有一个将B对象添加到A对象的方法。我想返回新添加的B对象的id。

例如：

public Long addBtoA(long aID){

            EntityTransaction tx = myDAO.getEntityManagerTransaction();

            tx.begin();
            A aObject = myDAO.load(aID);
            tx.commit();

            B bObject = new B();

            bObject.addB(bObject);

            tx.begin();
            myDAO.save(aObject);
            tx.commit();

            //Here I want to return the ID of the saved bObject.
            // After saving  aObject it's list of B objects has the newly added bObject with it's id. 
            // What is the best way to get its id?


}

Answer 1

我有一个将B对象添加到A对象的方法。我想返回新添加的B对象的id。

然后就这样做！在保留新B实例（并将更改刷新到数据库）之后，已分配其id，只需返回它。以下是一种说明此行为的测试方法：

@Test
public void test_Add_B_To_A() {
    EntityManagerFactory emf = Persistence.createEntityManagerFactory("MyPu");
    EntityManager em = emf.createEntityManager();
    em.getTransaction().begin();

    A a = em.find(A.class, 1L);

    B b = new B();
    A.addToBs(b); // convenient method that manages the bidirectional association

    em.getTransaction().commit(); // pending changes are flushed

    em.close();
    emf.close();

    assertNotNull(b.getId());
}

顺便说一句，你的代码有点混乱，每次与EM交互后你都不需要commit。

Answer 2

我认为接受的答案是正确的。见https://coderanch.com/t/628230/framework/Spring-Data-obtain-id-added

tldr; 您应该只为子项B创建一个存储库，这样您就可以完全独立于父项保存子项。获得保存后的B entity，然后将其与其父A相关联。

以下是一些示例代码，其中Todo为父级，Comment为子级。

@Entity
public class Todo {

    @OneToMany(mappedBy = "todo", cascade = CascadeType.ALL, orphanRemoval = true, fetch = FetchType.LAZY)
    private Set<Comment> comments = new HashSet<>();

    // getters/setters omitted.
}

@Entity
public class Comment {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @ManyToOne
    @JoinColumn(name = "todo_id")
    private Todo todo;

    // getters/setters omitted.
}

如果这是在spring数据中建模的，则创建2个存储库。 TodoRepository和CommentRepository Autowired in。

给定一个休息端点能够接收POST /api/todos/1/comments以将新评论与给定的待办事项ID相关联。

    @PostMapping(value = "/api/todos/{todoId}/comments")
    public ResponseEntity<Resource<Comment>> comments(@PathVariable("todoId") Long todoId,
                                                      @RequestBody Comment comment) {

        Todo todo = todoRepository.findOne(todoId);

        // SAVE the comment first so its filled with the id from the DB.
        Comment savedComment = commentRepository.save(comment);

        // Associate the saved comment to the parent Todo.
        todo.addComment(savedComment);

        // Will update the comment with todo id FK.
        todoRepository.save(todo);

        // return payload...
    }

如果您执行了以下操作，则保存提供的参数comment。获取新评论的唯一方法是遍历todo.getComments()并查找提供的comment，如果该集合是Set，则会感到烦人且不切实际。

  @PostMapping(value = "/api/todos/{todoId}/comments")
    public ResponseEntity<Resource<Comment>> comments(@PathVariable("todoId") Long todoId,
                                                      @RequestBody Comment comment) {

        Todo todo = todoRepository.findOne(todoId);

        // Associate the supplied comment to the parent Todo.
        todo.addComment(comment);

        // Save the todo which will cascade the save into the child 
        // Comment table providing cascade on the parent is set 
        // to persist or all etc.
        Todo savedTodo = todoRepository.save(todo);

        // You cant do comment.getId
        // Hibernate creates a copy of comment and persists it or something.
        // The only way to get the new id is iterate through 
        // todo.getComments() and find the matching comment which is 
        // impractical especially if the collection is a set. 

        // return payload...
    }

Answer 3

您应首先保留新创建的对象，然后将其添加到其容器中。另外，save的{{1}}方法返回新持久对象的标识符。所以你只需更新你的代码和/或你的DAO就像这样：

org.hibernate.Session

无论如何，对于具有生成ID的所有对象，您始终可以执行以下操作：

newObject.setContainer(container); // facultative (only if the underlying SGBD forbids null references to the container)
Long id = (Long) hibernateSession.save(newObject); // assuming your identifier is a Long
container.add(newObject);
// now, id contains the id of your new object

Answer 4

如果有人在以前的评论中找不到解决方案，另一个选择是添加

@GeneratedValue(strategy = yourChosenStrategy)

覆盖您持久存在的实体的ID（或其获取者的ID）。在这种情况下，当调用persist时，id将自动设置在持久化对象中。

希望它有所帮助！

以一对多关系获取持久子对象的ID

4 个答案: