JPA学习笔记六 单向一对多

表结构

CREATE TABLE `t_category` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8

CREATE TABLE `t_product` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  `typeid` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `FK_7kn4mhej5ccxp2ei4u2he6b8n` (`typeid`),
  CONSTRAINT `FK_7kn4mhej5ccxp2ei4u2he6b8n` FOREIGN KEY (`typeid`) REFERENCES `t_category` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8

实体类

package com.yjj.entity;

import javax.persistence.*;
import java.util.ArrayList;
import java.util.List;

/**
 * 单向一对多
 * t_category表字段，id，name
 */
@Table(name = "t_category")
@Entity
public class Category {
    @Id
    @GeneratedValue
    private Integer id;
    private String name;
    @JoinColumn(name = "typeId")
    @OneToMany
    private List<Product> products=new ArrayList<>();

    public Integer getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

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

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

    @Override
    public String toString() {
        return "Category{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", products=" + products +
                '}';
    }
}

package com.yjj.entity;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

/**
 * 单向一对多
 * t_product表字段，id，name，typeId
 */
@Table(name = "t_product")
@Entity
public class Product {
    @Id
    @GeneratedValue
    private Integer id;
    private String name;

    public Integer getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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



    @Override
    public String toString() {
        return "Product{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}

       Product product=new Product();
        product.setName("手机");
        Product product1=new Product();
        product1.setName("电脑");
        Category category=new Category();
        category.setName("数码");
        category.getProducts().add(product);
        category.getProducts().add(product1);

        manager.persist(product);
        manager.persist(product1);
        manager.persist(category);

输出

Hibernate: 
    insert 
    into
        t_product
        (name) 
    values
        (?)
Hibernate: 
    insert 
    into
        t_product
        (name) 
    values
        (?)
Hibernate: 
    insert 
    into
        t_category
        (name) 
    values
        (?)
Hibernate: 
    update
        t_product 
    set
        typeId=? 
    where
        id=?
Hibernate: 
    update
        t_product 
    set
        typeId=? 
    where
        id=?

查询一的一方

        Category category=manager.find(Category.class,1);
        System.out.println(category);

输出

  Hibernate: 
    select
        category0_.id as id1_0_0_,
        category0_.name as name2_0_0_ 
    from
        t_category category0_ 
    where
        category0_.id=?
Hibernate: 
    select
        products0_.typeId as typeId3_0_0_,
        products0_.id as id1_2_0_,
        products0_.id as id1_2_1_,
        products0_.name as name2_2_1_ 
    from
        t_product products0_ 
    where
        products0_.typeId=?
Category{id=1, name='数码', products=[Product{id=1, name='手机'}, Product{id=2, name='电脑'}]}

先看看数据库吧

image.png
image.png

删除

        Category category=manager.find(Category.class,1);
        manager.remove(category);

看输出

Hibernate: 
    select
        category0_.id as id1_0_0_,
        category0_.name as name2_0_0_ 
    from
        t_category category0_ 
    where
        category0_.id=?
Hibernate: 
    update
        t_product 
    set
        typeId=null 
    where
        typeId=?
Hibernate: 
    delete 
    from
        t_category 
    where
        id=?

image.png

删除Category后，Product并没有级联删除，而是把Product中typeid字段置为空，如果想要级联删除
@OneToMany(cascade=CascadeType.REMOVE)