多對多

在資料庫表格上要進行多對多對應，可以藉由一個中介表格來完成，也就是藉由多對一、一對多來完成多對多關聯。

多對多由於使用了中介表格，在查詢效率不彰，且在程式的物件模式上，多對多會使得物件與物件之間彼此依賴，並不是一個很好的設計方式，在設計上應避免使用多對多關係。

若要設計多對多關係，則可以使用@ManyToMany的標註，例如設計User類別如下：

User.java

package onlyfun.caterpillar;

import java.io.Serializable;
import java.util.Set;
import javax.persistence.CascadeType;
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.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Entity
@Table(name="T_USER")
public class User implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name="USER_ID")
    private Long id;
    private String name;
    private Long age;
    
    @ManyToMany(cascade=CascadeType.ALL)
    @JoinTable(
        name="T_USER_SERVER",
        joinColumns={@JoinColumn(name="USER_FK")},
        inverseJoinColumns={@JoinColumn(name="SERVER_FK")}
    )
    private Set<Server> servers;

    // 以下為 Getter、Setter
    ...
}

其中@JoinTable中name設定的是中介表格的名稱，並設定對應的欄位名稱。而Server可以如下設計：

Server.java

package onlyfun.caterpillar;

import java.util.Set;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Entity
@Table(name="T_SERVER")
public class Server {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name="SERVER_ID")
    private Long id;
    private String address;

    @ManyToMany(cascade=CascadeType.ALL, mappedBy="servers")
    private Set<User> users;

    // 以下是 Getter、Setter
    ...
}

這邊直接透過mappedBy屬性設定了雙向關聯，一個儲存時的例子如下：
        Server server1 = new Server();
        server1.setAddress("PC-219");
        server1.setUsers(new HashSet());

        Server server2 = new Server();
        server2.setAddress("PC-220");
        server2.setUsers(new HashSet());

        Server server3 = new Server();
        server3.setAddress("PC-221");
        server3.setUsers(new HashSet());

        User user1 = new User();
        user1.setName("caterpillar");
        user1.setServers(new HashSet());
        user1.setAge(new Long(35));

        User user2 = new User();
        user2.setName("momor");
        user2.setServers(new HashSet());
        user2.setAge(new Long(30));

// 多對多，互相參考
        user1.getServers().add(server1);
        user1.getServers().add(server2);
        user1.getServers().add(server3);
        server1.getUsers().add(user1);
        server2.getUsers().add(user1);
        server3.getUsers().add(user1);

        user2.getServers().add(server1);
        user2.getServers().add(server3);
        server1.getUsers().add(user2);
        server3.getUsers().add(user2);

        EntityManager entityManager =
                JPAUtil.getEntityManagerFactory().createEntityManager();
        EntityTransaction etx = entityManager.getTransaction();
        etx.begin();
        entityManager.persist(user1);
        entityManager.persist(user2);
        etx.commit();
        entityManager.close();

執行後資料庫的內容如下：

如果您使用以下方式進行查詢：
        EntityManager entityManager =
                JPAUtil.getEntityManagerFactory().createEntityManager();
        EntityTransaction etx = entityManager.getTransaction();
        etx.begin();
        user1 = entityManager.find(User.class, new Long(1));
        etx.commit();
        System.out.println(user1.getServers());
        entityManager.close();

若是使用Hibernate作為JPA的實作，則首先會先查詢出T_USER表格的內容，再透過left out join的方式查詢對應的T_SERVER中的資料，如以下所產生的SQL語句：

Hibernate:
    select
        user0_.USER_ID as USER1_0_0_,
        user0_.age as age0_0_,
        user0_.name as name0_0_
    from
        T_USER user0_
    where
        user0_.USER_ID=?
Hibernate:
    select
        servers0_.USER_FK as USER1_1_,
        servers0_.SERVER_FK as SERVER2_1_,
        server1_.SERVER_ID as SERVER1_1_0_,
        server1_.address as address1_0_
    from
        T_USER_SERVER servers0_
    left outer join
        T_SERVER server1_
            on servers0_.SERVER_FK=server1_.SERVER_ID
    where
        servers0_.USER_FK=?