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();
     }