GORM 关联与迁移

第 11 章 · GORM 关系与 GORM 链式 API

本章目标：为 shop-db 配置 @OneToMany / @ManyToOne 双向关系；使用 GORM 链式 API 与 Criteria API 编写类型安全查询；识别并缓解 N+1 问题；掌握 JOIN FETCH 与 gorm.ModelGraph 入门；延续 slug + is_published + price（分） 业务查询。

学时建议：6～7 小时（含 3 小时关系映射跟练）

前置：完成 go-database ch10（实体与 gorm.DB）；gin-web ch05 §5.6（N+1 预习）。

11.1 场景说明：分类与商品的 1:N

电商列表页需展示「分类名 + 商品标题 + 价格」。ch10 的 Product 仅有 categoryId Long，无法 product.getCategory().getName()。本章建立对象图：

Category (1) ──────< Product (N)
   id                    category_id FK
   name                  title, slug, price, is_published

11.2 @ManyToOne 与 @OneToMany 双向映射

11.2.1 Product 端（多的一方，持 FK）

gorm.Model
@Table(name = "products")
public class Product {

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

    @ManyToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "category_id", nullable = false)
    private Category category;

    @Column(nullable = false, length = 128)
    private String title;

    @Column(nullable = false, unique = true, length = 128)
    private String slug;

    @Column(nullable = false)
    private long price;  // 分

    @Column(name = "is_published", nullable = false)
    private boolean published = false;

    @Column(nullable = false)
    private int stock = 0;

    protected Product() {}

    public Product(String title, String slug, long priceCents, Category category) {
        this.title = title;
        this.slug = slug;
        this.price = priceCents;
        this.category = category;
    }

    public Category getCategory() { return category; }
    public String getSlug() { return slug; }
    public long getPrice() { return price; }
    public boolean isPublished() { return published; }
}

11.2.2 Category 端（一的一方，反向集合）

gorm.Model
@Table(name = "categories")
public class Category {

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

    @Column(nullable = false, length = 64)
    private String name;

    @Column(nullable = false, unique = true, length = 64)
    private String slug;

    @OneToMany(mappedBy = "category", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Product> products = new ArrayList<>();

    protected Category() {}

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

    public void addProduct(Product product) {
        products.add(product);
        product.setCategory(this);
    }

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

Product.setCategory(Category) 为 package-private，仅 addProduct 调用，防止单向设置导致 FK 不一致。

11.2.3 与 gin-web ch05 对照

11.3 级联（Cascade）与 orphanRemoval

@OneToMany(mappedBy = "category", cascade = CascadeType.ALL, orphanRemoval = true)
private List<Product> products = new ArrayList<>();

shop-db 建议：

• 删分类：DB 层 ON DELETE RESTRICT，应用层禁止 remove(category) 若有商品。
• orphanRemoval：仅从集合 remove(product) 时删商品行，符合「从分类移除即删 SKU」后台操作（需权限控制）。

11.4 GORM 链式 API 基础

GORM 链式 API 面向实体，非表：

// 按 slug 查上架商品
Product p = em.createQuery("""
    SELECT p FROM Product p
    WHERE p.slug = :slug AND p.published = true
    """, Product.class)
    .setParameter("slug", "wireless-mouse")
    .getSingleResult();

// 分类下已上架商品，按价升序
List<Product> list = em.createQuery("""
    SELECT p FROM Product p
    JOIN p.category c
    WHERE c.slug = :catSlug AND p.published = true
    ORDER BY p.price ASC
    """, Product.class)
    .setParameter("catSlug", "digital")
    .getResultList();

11.4.1 分页

List<Product> page = em.createQuery("""
    SELECT p FROM Product p WHERE p.published = true ORDER BY p.createdAt DESC
    """, Product.class)
    .setFirstResult(0)
    .setMaxResults(20)
    .getResultList();

11.4.3 GORM 链式 API 速查

分页用 setFirstResult / setMaxResults；聚合返回 Long.class。

11.5 JOIN FETCH 解决 N+1

11.5.1 N+1 现象

List<Product> products = em.createQuery(
    "SELECT p FROM Product p WHERE p.published = true", Product.class)
    .getResultList();