package com.jpa.yingshe;

import javax.persistence.*;
import java.util.HashSet;
import java.util.Set;

@Table(name = "JPA_CUTOMERS")
@Entity
public class Customer {

    private Integer id;
    private String lastName;

    private String email;

    private Set<Order> orders = new HashSet<>();

    public Customer() {}

    public Customer(String lastName) {
        this.lastName = lastName;
    }

    @GeneratedValue(strategy = GenerationType.AUTO)
    @Id
    public Integer getId() {
        return id;
    }

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

    @Column(name = "LAST_NAME", length = 50, nullable = false)
    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    @JoinColumn(name="CUSTOMER_ID")
    @OneToMany(cascade={CascadeType.REMOVE})
    public Set<Order> getOrders() {
        return orders;
    }

    public void setOrders(Set<Order> orders) {
        this.orders = orders;
    }
}

package com.jpa.yingshe;

import javax.persistence.*;

@Table(name = "JPA_ORDERS")
@Entity
public class Order {

    private Integer id;
    private String orderName;

    private Customer customer;

    @GeneratedValue
    @Id
    public Integer getId() {
        return id;
    }

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

    @Column(name = "ORDER_NAME")
    public String getOrderName() {
        return orderName;
    }

    public void setOrderName(String orderName) {
        this.orderName = orderName;
    }

    @JoinColumn(name="CUSTOMER_ID")
    @ManyToOne(fetch=FetchType.LAZY)
    public Customer getCustomer() {
        return customer;
    }

    public void setCustomer(Customer customer) {
        this.customer = customer;
    }
}

package jpa.test;

import com.jpa.yingshe.Customer;
import com.jpa.yingshe.Order;

import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;

public class JPAyingshe {
    private EntityManagerFactory entityManagerFactory;
    private EntityManager entityManager;
    private EntityTransaction transaction;

    @Before
    public void init() {
        entityManagerFactory = Persistence.createEntityManagerFactory("jpaname");
        entityManager = entityManagerFactory.createEntityManager();
        transaction = entityManager.getTransaction();
        transaction.begin();
    }

    @After
    public void destroy() {
        transaction.commit();
        entityManager.close();
        entityManagerFactory.close();
    }

    // 若是双向 1-n 的关联关系, 执行保存时
    // 若先保存 n 的一端, 再保存 1 的一端, 默认情况下, 会多出 2n 条 UPDATE 语句
    // 若先保存 1 的一端, 则会多出 n 条 UPDATE 语句
    // 在进行双向 1-n 关联关系时, 建议使用 n 的一方来维护关联关系, 而 1 的一方不维护关联系, 这样会有效的减少 SQL 语句
    // 注意: 若在 1 的一端的 @OneToMany 中使用 mappedBy 属性, 则 @OneToMany 端就不能再使用 @JoinColumn 属性了
    @Test
    public void testOneToManyPersist() {
        Customer customer = new Customer();
        customer.setEmail("mm@163.com");
        customer.setLastName("MM");

        Order order1 = new Order();
        order1.setOrderName("O-MM-1");

        Order order2 = new Order();
        order2.setOrderName("O-MM-2");

        //建立关联关系
        customer.getOrders().add(order1);
        customer.getOrders().add(order2);

        order1.setCustomer(customer);
        order2.setCustomer(customer);

        entityManager.persist(order1);
        entityManager.persist(order2);
        //执行保存操作
        entityManager.persist(customer);
    }
}

// 若在 1 的一端的 @OneToMany 中使用 mappedBy 属性, 则 @OneToMany 端就不能再使用 @JoinColumn 属性
// @JoinColumn(name="CUSTOMER_ID")
@OneToMany(mappedBy="customer")
public Set<Order> getOrders() {
    return orders;
}

@Test
public void testOneToManyPersist() {
    Customer customer = new Customer();
    customer.setEmail("mm@163.com");
    customer.setLastName("MM");

    Order order1 = new Order();
    order1.setOrderName("O-MM-1");

    Order order2 = new Order();
    order2.setOrderName("O-MM-2");

    //建立关联关系
    customer.getOrders().add(order1);
    customer.getOrders().add(order2);

    order1.setCustomer(customer);
    order2.setCustomer(customer);

    //执行保存操作
    entityManager.persist(customer);

    entityManager.persist(order1);
    entityManager.persist(order2);
}