JPA学习笔记七 双向一对多(多对一)

双向一对多和双向多对一是一样的，就是两方都维护对方嘛

package com.yjj.entity;

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

/**
 * author
 */
@Entity
@Table(name = "t_product")
public class Product {
    @Id
    @GeneratedValue
    private Long id;
    private String name;
    // 多对一：optional=false表示外键type_id不能为空
    @ManyToOne(optional = true)
    @JoinColumn(name = "type_id")
    private ProductType type;

    public Product() {
    }

    public Product(String name, ProductType type) {
        this.name = name;
        this.type = type;
    }

    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;
    }

    public ProductType getType() {
        return type;
    }

    public void setType(ProductType type) {
        this.type = type;
    }

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

package com.yjj.entity;
import javax.persistence.*;
import java.util.ArrayList;
import java.util.List;


@Entity
@Table(name = "t_product_type")
public class ProductType {
    @Id
    @GeneratedValue
    private Long id;
    private String name;
    @OneToMany
    @JoinColumn(name="type_id")
    private List<Product> list = new ArrayList<Product>();

    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;
    }

    public List<Product> getList() {
        return list;
    }

    public void setList(List<Product> list) {
        this.list = list;
    }

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

  ProductType type=new ProductType();
        type.setName("数码");
        Product product=new Product();
        product.setName("手机");
        product.setType(type);
        Product product1=new Product();
        product1.setName("电脑");
        product1.setType(type);
        type.getList().add(product);
        type.getList().add(product1);
        entityManager.persist(type);
        entityManager.persist(product);
        entityManager.persist(product1);

先保存一的一方，再保存多的一方

Hibernate: 
    insert 
    into
        t_product_type
        (name) 
    values
        (?)
Hibernate: 
    insert 
    into
        t_product
        (name, type_id) 
    values
        (?, ?)
Hibernate: 
    insert 
    into
        t_product
        (name, type_id) 
    values
        (?, ?)
Hibernate: 
    update
        t_product 
    set
        type_id=? 
    where
        id=?
Hibernate: 
    update
        t_product 
    set
        type_id=? 
    where
        id=?

换换位置

        entityManager.persist(product);
        entityManager.persist(product1);
        entityManager.persist(type);

再看输出sql

Hibernate: 
    insert 
    into
        t_product
        (name, type_id) 
    values
        (?, ?)
Hibernate: 
    insert 
    into
        t_product
        (name, type_id) 
    values
        (?, ?)
Hibernate: 
    insert 
    into
        t_product_type
        (name) 
    values
        (?)
Hibernate: 
    update
        t_product 
    set
        name=?,
        type_id=? 
    where
        id=?
Hibernate: 
    update
        t_product 
    set
        name=?,
        type_id=? 
    where
        id=?
Hibernate: 
    update
        t_product 
    set
        type_id=? 
    where
        id=?
Hibernate: 
    update
        t_product 
    set
        type_id=? 
    where
        id=?

多了两个update，所以知道怎么做了嘛？嗯？小傻瓜~
我们还可以用mappedBy注解，让一的一方来维护关系。

@OneToMany(mappedBy="user")
private List<Order> orders = new ArrayList<>();

但是使用了mapperBy注解后就不能用JoinColumn注解了