加入实体之间的列得到 NULL 值而不是父实体 ID 号

Question

I am Using Spring Boot on Java to create user's order on his checkout. A new Orders object is created which has a Linked Set of Items. Those items are user's cart contents.

Order is created, but its set of Items is null. The set size is 0. I checked that in JUnit tests. Can you help me to find out what is wrong? Maybe I have defined entities incorrectly? Have a look at the picture of the database:

And check the entities, Orders:

@Entity
public class Orders {
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    
    @NotEmpty
    @DateTimeFormat(pattern = "yyyy-MM-dd HH:mm:ss")
    private LocalDateTime submitedAt;
    
    @NotEmpty
    private String orderName;
    
    @NotEmpty
    @Column(name="`User`")
    private String username;
    
    @Enumerated(EnumType.STRING)
    @Column
    private OrderStatus status;
    
    @OneToMany(mappedBy = "orders", cascade = { CascadeType.ALL}, fetch = FetchType.LAZY)
    private Set<Item> items;

Item:

@Entity
public class Item {
    
    @Id
    private Integer id;
    
    @Column(name="`NAME`")
    private String dishName;
    
    @Column(name = "`DESCRIPTION`", length = 2000)
    private String dishDescription;
    
    @Column(name = "`QUANTITY`")
    private Integer quantityInCart;
    
    @Column(name = "`USER`")
    private String username;
    
    @ManyToOne(cascade = { CascadeType.PERSIST, CascadeType.MERGE, CascadeType.DETACH, CascadeType.REFRESH })
    @JoinColumn(name = "ORDERS_ID")
    private Orders orders;

How to do entities relation correctly? Should it be one direction or bi-directional relationship? What are differences of these relations? And what kind of relationship I should use? Why? I was doing JUnit tests for the Orders service methods. It turns out that it can create orders. And Order items from user's cart. But when it is time to show order (GetMapping) then it returns Orders entity with empty items set. I think it happens because JPA cannot find foreign key of items for its designated order. It is null. Why is it null?

And this is the service method that creates such order by user request:

@Transactional
public ResponseEntity<String> createOrder (String username) {
    User user = userService.findByUsername(username);
    List<CartItem> items = cartRepo.findByUser(user);

    if(items.size() > 0) {
    Orders newOrder = new Orders();
    Set<Item> orderItems = new LinkedHashSet<>();
            
    for(CartItem item : items) {
// new Item(Integer id, String dishName, String dishDescription, Integer quantityInCart, String username)
        Item orderItem = new Item(item.getId(), item.getDish().getName(), 
                                      item.getDish().getDescription(), item.getQuantity(), item.getUser().getUsername());           
        orderItems.add(orderItem);
    }
    newOrder.setItems(orderItems);
    newOrder.setOrderName(user.getUsername()+"'s order");
    newOrder.setStatus(OrderStatus.SUBMIT);
    newOrder.setSubmitedAt();
    newOrder.setUsername(username);
    orderDao.save(newOrder);
    cartService.removeAllUserProducts(username);
    LOG.info("[{}]: A new order is created successfully.", username);
    return new ResponseEntity<String>("A new order is created successfully.", HttpStatus.CREATED);
    }
//...
}

I tried to do one direction relationship for other entities and it really created foreign keys on joined column fields. But I want to find out why my bidirectional way of joining is wrong. Maybe someone who really knows can explain.

Answer 1

The Order class should be like this:

@Entity

public class Orders {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Integer id;

@NotEmpty
@DateTimeFormat(pattern = "yyyy-MM-dd HH:mm:ss")
private LocalDateTime submitedAt;

@NotEmpty
private String orderName;

@NotEmpty
@Column(name="`User`")
private String username;

@Enumerated(EnumType.STRING)
@Column
private OrderStatus status;

@OneToMany(cascade = { CascadeType.ALL}, fetch = FetchType.LAZY, orphanRemoval = true)
@JoinColumn(name="ORDERS_ID")
private Set<Item> items;

And Item class without Orders class and its ManyToOne relationship.

Now relationship is unidirectional. Item entity has foreign keys column name ORDERS_ID that has id's of Orders for which Items belong.