实现Entity Bean的一对多(one-to-many)映射
EJB3的一对多映射使用@OneToMany来设置,如果是双向的一对多映射,在many方需要使用@ManyToOne 设置。在本书中给出两个表,其他一个表t_customers在上一篇文章中已给出了,另一个表的结构如图1所 示。
图1 t_orders表
t_customers和t_orders表是一对多关系,一个Customer可能有多个Order,而一个Order只能有一个 Customer。
在Customer类中需要定义一个集合类型的属性,用来保存多个Order对象,Customer类的代码如下:
package entity;import java.util.Collection;import javax.persistence.CascadeType;import javax.persistence.Entity;import javax.persistence.FetchType;import javax.persistence.GeneratedValue;import javax.persistence.GenerationType;import javax.persistence.Id;import javax.persistence.JoinColumn;import javax.persistence.JoinTable;import javax.persistence.ManyToMany;import javax.persistence.OneToMany;import javax.persistence.OneToOne;import javax.persistence.PrimaryKeyJoinColumn;import javax.persistence.Table;@Entity@Table(name = "t_customers")public class Customer{ private int id; private String name; private Referee referee; private Collection<Order> orders; @OneToMany(mappedBy = "customer", cascade = CascadeType.ALL) public Collection<Order> geTorders() { return orders; } public void seTorders(Collection<Order> orders) { this.orders = orders; } @Id @GeneratedValue(strategy = GenerationType.IDENTITY) public int getId() { return id; } ... ...}
其中@OneToMany的mappedBy属性指定了Order类中获得Customer对象的属性名。Order类的代码如下:
package entity;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.GenerationType;import javax.persistence.Id;import javax.persistence.JoinColumn;import javax.persistence.JoinColumns;import javax.persistence.JoinTable;import javax.persistence.ManyToOne;import javax.persistence.Table;@Entity@Table(name = "t_orders")public class Order{ private int id; private String productId; private int count; private Customer customer; @ManyToOne @JoinColumn(name = "customer_id") public Customer getCustomer() { return customer; } public void setCustomer(Customer customer) { this.customer = customer; } @Id @GeneratedValue(strategy = GenerationType.IDENTITY) public int getId() { return id; } ... ...}
其中@JoinColumn注释的name属性指定t_orders表中用于连接t_customers表的外键名。
可以使用下面的代码进行测试:
Customer customer = new Customer(); customer.setName("微软"); List<Order> rders = new ArrayList<Order>(); Order rder =new Order(); order.setProductId("1234"); order.setCount(20); order.setCustomer(customer); orders.add(order); rder = new Order(); order.setProductId("4321"); order.setCount(12); order.setCustomer(customer); orders.add(order); customer.seTorders(orders); em.persist(customer);
除此之外,还可以使用@JoinTable指定连接表来映射one-to-many关系。连接表的结构如图2所示。
图2 t_customers_orders表
首选需要将Customer类的geTorders方法修改成下的形式:
@OneToMany @JoinTable(name = "t_customers_orders", joinColumns = @JoinColumn(name = "t_customers_id", referencedColumnName = "id"), inverseJoinColumns = @JoinColumn(name = "orders_id", referencedColumnName = "id")) public Collection<Order> geTorders() { return orders; }
其中name属性指定图2所示的连接表的名称。joinColumns指定了t_customers表和t_customers_orders 表中相连接的字段。inverseJoinColumns指定了t_orders表和t_customers_orders表中相连接的字段。这 时t_orders表中的customer_id字段就不再需要了。因此,需要将Order表中的customer属性(getter和 setter方法)去掉。
在持久化Customer和Order对象时,需要对Order对象进行持久化后才能对Customer对象进行持久化。
见所未见,闻所未闻。