Hibernate 数据库关系注解

Question

我已经为此苦苦挣扎了很长时间。我有一个包含两个表“产品”和“类别”的数据库

CREATE TABLE `product` (
  `idproduct` int NOT NULL AUTO_INCREMENT,
  `idcategory` int DEFAULT NULL,
  `product_name` varchar(255) DEFAULT NULL,
  `product_category` varchar(255) DEFAULT NULL,
  `product_description` varchar(255) DEFAULT NULL,
  `created_at` timestamp NULL DEFAULT CURRENT_TIMESTAMP,
  `updated_at` timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
   PRIMARY KEY (`idproduct`),
   KEY `fkcat` (`idcategory`),
   CONSTRAINT `fkcat` FOREIGN KEY (`idcategory`) REFERENCES `categories` (`idcategory`)
 ) ENGINE=InnoDB AUTO_INCREMENT=149 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

CREATE TABLE `categories` (
  `idcategory` int NOT NULL AUTO_INCREMENT,
  `category_name` varchar(255) NOT NULL,
  `category_description` varchar(255) DEFAULT NULL,
  `created_at` timestamp NULL DEFAULT CURRENT_TIMESTAMP,
  `updated_at` timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
   PRIMARY KEY (`idcategory`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=20 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

现在我正在尝试获取一个休眠连接查询，以便我可以检索让我们说 产品名称和类别名称

一个产品只属于一个类别（例如，如果产品是“黑色 T 恤”，那么“idcategory”列的值将是 2。这是由外键强制执行的。 表类别条目可以与多个产品相关联（例如，“category_name”= 2 可以与多个产品相关联。

如何在休眠实体中实现这种设计？我试过了，但没有用...

@Entity
@Table(name = "product")
public class Product implements Serializable{
    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "idproduct")
    private int idproduct;
    @Column(name = "idcategory")
    private int idcategory;
    @Column(name = "product_name")
    private String productName;
    @Column(name = "product_description")
    private String productdescription;
    @Column(name = "product_category")
    private String productcategory;
    @OneToMany(targetEntity = Categories.class, cascade = CascadeType.ALL)
    @JoinColumn(name = "idcategory",referencedColumnName="idcategory")
    private List<Categories> category;

---

@Entity
@Table(name = "categories")
public class Categories {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "idcategory")
    private int idcategory;
    @Column(name = "category_name")
    private String category_name;
    @Column(name = "category_description")
    private String category_description;

查询是

SELECT p, c FROM Product p INNER JOIN p.category c

Answer 1

这是不对的

 @OneToMany(targetEntity = Categories.class, cascade = CascadeType.ALL)
 @JoinColumn(name = "idcategory",referencedColumnName="idcategory")
 private List<Categories> category;

产品不能有很多类别...实际上是相反的->

@Entity
@Table(name = "categories")
public class Categories {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "idcategory")
    private int idcategory;

    @Column(name = "category_name")
    private String category_name;

    @Column(name = "category_description")
    private String category_description;

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

和产品

@Entity
@Table(name = "product")
public class Product implements Serializable{
    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "idproduct")
    private int idproduct;
    @Column(name = "idcategory")
    private int idcategory;
    @Column(name = "product_name")
    private String productName;
    @Column(name = "product_description")
    private String productdescription;
    @Column(name = "product_category")
    private String productcategory;
    @ManyToOne
    private Categories categories;

建议：将Categories 重命名为Category