我在尝试用Spring Boot构建一个电商API。我使用Spring Data JPA来建模和设计关系，但不知为何，我无法实现这个一对多关系。

我有一个Category实体

package xyz.rahamatj.daraz_like.models;

import com.fasterxml.jackson.annotation.JsonManagedReference;
import jakarta.persistence.*;

import java.util.List;

@Entity
@Table(name = "categories")
public class Category {

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "categories_sequence_gen")
    @SequenceGenerator(name = "categories_sequence_gen", sequenceName = "categories_id_seq", allocationSize = 1)
    private Long id;
    private String name;

    @OneToMany(mappedBy = "category", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private List<Product> products;

    public Category() {
    }

    public Category(String name) {
        this.name = name;
    }

    public Category(Long id, String name) {
        this.id = id;
        this.name = name;
        this.products = products;
    }

    public Category(String name, List<Product> products) {
        this.name = name;
        this.products = products;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @OneToMany(mappedBy = "category")
    private List<Product> products;

    public List<Product> getProducts() {
        return products;
    }

    public void setProducts(List<Product> products) {
        this.products = products;
    }
}

并且我有一个Product实体

package xyz.rahamatj.daraz_like.models;

import com.fasterxml.jackson.annotation.JsonBackReference;
import jakarta.persistence.*;
import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Data;
import lombok.NoArgsConstructor;

@Entity
@Table(name = "products")
public class Product {

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "products_sequence_gen")
    @SequenceGenerator(name = "products_sequence_gen", sequenceName = "products_id_seq", allocationSize = 1)
    private Long id;
    private String name;
    private double rating;
    private double price;
    private String description;

    @ManyToOne
    @JoinColumn(name = "category_id")
    private Category category;

    public Product() {
    }

    public Product(String name, double rating, double price, String description) {
        this.name = name;
        this.rating = rating;
        this.price = price;
        this.description = description;
//        this.category = category;
    }

    public Product(Long id, String name, double rating, double price, String description) {
        this.id = id;
        this.name = name;
        this.rating = rating;
        this.price = price;
        this.description = description;
//        this.category = category;
    }
}

现在我无法实现从Category到 Product的一对多关系。

解决方案

正如你在注释中所说，你得到的输出类似于：

category: {
    id: 1,
    title: "test",
    products: []
}

原因在于这一行：

@OneToMany(mappedBy = "category", cascade = CascadeType.ALL, fetch = FetchType.LAZY)

你对Product使用的fetch type为 LAZY。这意味着它不会从数据源隐式获取。你必须显式地从数据源获取它们。

因此，每当你从你的数据源获取一个Category时，产品都不会被包含在结果中，正是因为这个原因。 如果你也希望在响应中包含产品，并且不想进行显式调用，请设置 fetch = FetchType.EAGER。这将包含与该分类相关的所有产品。