JPA-一对多关系

JPA中，一对多关系使用@OneToMany标示

关系维护端：

package com.yl.demo1.bean.oneTomany;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;


@Entity
public class OrderItem {
    
    private Integer id;
    private String productName;
    private Float sellPrice;
    private Order order;
    
    @Id @GeneratedValue
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    @Column(length=40, nullable=false)
    public String getProductName() {
        return productName;
    }
    public void setProductName(String productName) {
        this.productName = productName;
    }
    @Column(nullable=false)
    public Float getSellPrice() {
        return sellPrice;
    }
    public void setSellPrice(Float sellPrice) {
        this.sellPrice = sellPrice;
    }
    /**
     * optional代表属性是否可选,
     * true--代表字段可为空 
     * false--代表字段不允许为null
     * 
     * @JoinColumn---指定外键的名称
     */
    @ManyToOne(cascade={CascadeType.MERGE, CascadeType.REFRESH}, optional=true)
    @JoinColumn(name="order_id")
    public Order getOrder() {
        return order;
    }
    public void setOrder(Order order) {
        this.order = order;
    }
    
    
}

关系被维护端：

package com.yl.demo1.bean.oneTomany;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;
/**
 * JPA中，1-m的双向关系中，多的一方为关系维护端，关系维护端服务外键记录的更新，关系被维护端是没有权利更新外键记录的
 * 
 * 
 */
@Entity
@Table(name="orders")
public class Order {
    private String orderId;
    private Float amount = 0f;
    private Set<OrderItem> items = new HashSet<OrderItem>();
    
    @Id @Column(length=12)
    public String getOrderId() {
        return orderId;
    }
    public void setOrderId(String orderId) {
        this.orderId = orderId;
    }
    @Column(nullable=false)
    public Float getAmount() {
        return amount;
    }
    public void setAmount(Float amount) {
        this.amount = amount;
    }
    /**级联操作
     * CascadeType.REFRESH--select order, select OrderItem
     * CascadeType.PERSIST--insert into Order, insert into OrderItem
     * CascadeType.MERGE----当Order处于游离状态时，对Order更新，也会更新OrderItem--update Order, for(update OrderItem)
     * CascadeType.REMOVE---delete Order, delete OrderItem
     * 可以采用CascadeType.ALL包括上述四种.
     * 上述四种方法只有调用相应的方法时才会触发。即EntityManager.refresh(),EntityManager.persist(),EntityManager.merge(), EntityManager.remove()
     *
     *   *ToMany的fetch默认值为FetchType.LAZY
     *   *ToOne的fetch默认值为FetchType.EAGER
     *   
     *   
     *   mappedBy出现在关系的被维护端,它指定在关系维护端是由哪个属性进行维护关系
     */
    @OneToMany(cascade={CascadeType.REFRESH, CascadeType.PERSIST}
            , fetch=FetchType.LAZY
            , mappedBy="order")
    public Set<OrderItem> getItems() {
        return items;
    }
    public void setItems(Set<OrderItem> items) {
        this.items = items;
    }
    
    
    public void addOrderItem(OrderItem orderItem) {
        orderItem.setOrder(this);
        this.items.add(orderItem);
    }
    
}

 常见操作:

@Test
    public void save() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();//事务开始
        
        Order order = new Order();
        order.setAmount(34f);
        order.setOrderId("999");
        
        OrderItem orderItem1 = new OrderItem();
        orderItem1.setProductName("football");
        orderItem1.setSellPrice(90f);
        
        OrderItem orderItem2 = new OrderItem();
        orderItem2.setProductName("basketball");
        orderItem2.setSellPrice(100f);
        
        order.addOrderItem(orderItem1);
        order.addOrderItem(orderItem2);

        em.persist(order);
        
        em.getTransaction().commit();
        em.close();
        factory.close();
    }