如何从对象的 OneToMany 集合中获取对象？

Question

我有一个Order 实体和OrderProduct。我想在前端显示订单详细信息，当然还有订单产品。那么如何在OrderProduct JSON 中获取产品对象。我在产品数组中缺少产品对象。我不再需要订单对象，我认为这将是一个无限递归的东西。 :)

我的Order 实体：

@Entity
@Getter
@Setter
@Table(name ="orders")
public class Order{

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    public Long id;

    private BigDecimal totalPrice;

    @OneToMany(mappedBy = "order", fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    @JsonManagedReference(value="orders")
    private List<OrderProduct> products = new ArrayList<>();

    private int userId;

    @DateTimeFormat(pattern="dd/MM/yyyy")
    private Date date = new Date();

    @DateTimeFormat(pattern="dd/MM/yyyy")
    private Date deliveryDate;

    @Enumerated(EnumType.STRING)
    private OrderType orderType;

}

我的OrderProduct 实体：

@Entity
@Setter
@Getter
public class OrderProduct {

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

    @ManyToOne(fetch = FetchType.EAGER)
    @JsonBackReference(value="product")
    @JoinColumn(name = "product_id")
    private Product product;

    @ManyToOne
    @JsonBackReference(value="orders")
    @JoinColumn(name = "order_id")
    private Order order;

    private Integer quantity;
}

产品实体：

@Entity
@Getter
@Setter
public class Product {

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

    @Column(unique = true)
    private String name;

    private double price;

    @OneToMany(mappedBy = "product", cascade = CascadeType.ALL)
    @JsonManagedReference(value="ingredients")
    private List<Ingredient> ingredients = new ArrayList<>();

    @OneToMany(mappedBy = "product",fetch = FetchType.EAGER)
    @JsonManagedReference(value="product")
    private List<OrderProduct> products = new ArrayList<>();

    private String fileName;

}

Answer 1

这有助于用

注释您的一个实体类

@JsonIdentityInfo(
    property = "id",
    generator = ObjectIdGenerators.PropertyGenerator.class
)

每次 JSON 序列化循环时，对象数据都会被替换为对象 id 或实体字段供您选择。

Answer 2

您可以使用@JsonViewannotation 来定义需要序列化为 JSON 的字段

它是如何工作的：

1. 您需要使用接口定义类。例如：

   public class SomeView {
   
       public interface  id {}
   
       public interface CoreData extends id {}
   
       public interface FullData extends CoreData {}
   
   }
   

2. 用@JsonView(<some interface.class>)标记实体字段

   public class User {
   
       @Id
       @GeneratedValue(strategy = GenerationType.AUTO)
       @JsonView(SomeView.id.class)
       private Long id;
   
       @Column(nullable = false)
       @JsonView(SomeView.CoreData.class)
       private String username;
   
       @Column(nullable = false)
       @JsonView(SomeView.FullData.class)
       private String email;
   }
   

3. 用@JsonView(<some interface.class>)注释端点

    @GetMapping()
    @JsonView(<some interface.class>)
    public User getUser() {
        return <get user entity somwhere>
    }
   

如果@JsonView(SomeView.id.class) 你会得到这个 JSON：

    {
        id: <some id>
    }

万一@JsonView(SomeView.CoreData.class)：

    {
        id: <some id>,
        username: <some username>
    }

万一@JsonView(SomeView.FullData.class)：

    {
        id: <some id>,
        username: <some username>,
        email: <some email>
    }

@JsonView 也适用于嵌入对象，您可以使用多个视图类注释一个字段 - @JsonView({SomeView.FullData.class, SomeOtherView.OtherData.class})

在你的情况下，我认为你应该注释你需要的所有字段，除了：

@OneToMany(mappedBy = "product",fetch = FetchType.EAGER)
@JsonManagedReference(value="product")
private List<OrderProduct> products = new ArrayList<>();

在Product

避免循环序列化

或者作为替代方案，您可以只使用 DTO 类或手动将 oject 序列化为 JSON (https://thepracticaldeveloper.com/java-and-json-jackson-serialization-with-objectmapper/)

Answer 3

这可以通过我的图书馆beanknife来完成

// This configure generate a class named ProductInfo which has the same shape with Product without property "products"
@ViewOf(value = Product.class, genName="ProductInfo", includePattern = ".*", excludes = {"products"})
class ProductInfoConfigure {}

// This configure generate a class named OrderProductRelation with the same shape of OrderProduct.
// But it has not order property and the type of its product property is change to ProductInfo generated above.
@ViewOf(value = OrderProduct.class, genName="OrderProductRelation", includePattern = ".*", excludes = {"order"})
class OrderProductRelationConfigure {
    @OverrideViewProperty("product")
    private ProductInfo product;
}

// This configure generate a class named OrderDetail with the same shape of Order. 
// But the type of its products property is change to List<OrderProductRelation>
@ViewOf(value = Order.class, genName="OrderDetail", includePattern = ".*")
class OrderDetailConfigure {
    @OverrideViewProperty("products")
    private List<OrderProductRelation> products;
}

将生成这些类：

class ProductInfo {
    private Long id;
    private String name;
    private double price;
    private List<Ingredient> ingredients; // it is not processed because you have not provide the class Ingredient
    private String fileName;
}

public class OrderProductRelation {
    private Long id;
    private ProductInfo product;
    private Integer quantity;
}

public class OrderDetail {
    public Long id;
    private BigDecimal totalPrice;
    private List<OrderProductRelation> products;
    private int userId;
    private Date date = new Date();
    private Date deliveryDate;
    private OrderType orderType;
}

然后

Order order = ...
OrderDetail orderDetail = OrderDetail.read(order);
// serialize the otherDetail instead of order.

List<Order> orders = ...
List<OrderDetail> orderDetails = OrderDetail.read(orders);
// serialize the orderDetails instead of orders.

可能的问题：

1. 我不使用 Lombok，因此可能需要调整 Lombok，因为它会动态更改字节码。但这不是什么大问题，如果有人提交问题并提供足够的用例，我会尝试调整它。
2. 生成的类不继承原类的注解。在下一个版本中，我将提供一个 sulotion。此时，作为一种解决方法，我们可以使用自定义方法手动转换属性。比如

@ViewOf(value = Order.class, genName="OrderDetail", includePattern = ".*")
class OrderDetailConfigure {
    @OverrideViewProperty("products")
    private List<OrderProductRelation> products;
    @OverrideViewProperty("orderType")
    public static String orderType(Order source) {
        return source.getOrder().name();
    }
}

生成的类会改为

public class OrderDetail {
    public Long id;
    private BigDecimal totalPrice;
    private List<OrderProductRelation> products;
    private int userId;
    private Date date = new Date();
    private Date deliveryDate;
    private String orderType;
}

更新 版本 1.2.0 发布。添加注解继承支持。

@ViewOf(value = Order.class, genName="OrderDetail", includePattern = ".*")
@UseAnnotation({DateTimeFormat.class, Enumerated.class, JsonProperty.class})
class OrderDetailConfigure {
    @OverrideViewProperty("products")
    private List<OrderProductRelation> products;
}

生成

public class OrderDetail {
    public Long id;
    private BigDecimal totalPrice;
    private List<OrderProductRelation> products;
    private int userId;
    @DateTimeFormat(pattern="dd/MM/yyyy")
    private Date date;
    @DateTimeFormat(pattern="dd/MM/yyyy")
    private Date deliveryDate;
    @Enumerated(EnumType.STRING)
    private OrderType orderType;
}