Hibernate 多對多資料更新範例

因同學在這方面比較有問題，才有這份文件。

以下是多對多使用 Java 物件實作範例:

情境

假設這是一個朋友與群組的多對多關係，一個群組可以存多個朋友，朋友也可以屬於多個群組。

SQL Table 設計如下(本範例為 MSSQL 語法)

SQL 關聯圖:

use hibernatedb
GO

create table friends(
friends_id int not null primary key identity(1,1),
friendName nvarchar(50) not null
)
GO

create table groups(
groups_id int not null primary key identity(1,1),
groupName nvarchar(50) not null
)
GO

create table friend_group(
fg_id int not null primary key identity(1,1),
fk_friend_id int not null references friends(friends_id),
fk_group_id int not null references groups(groups_id)
)

Java Bean 設置

1. MyGroup.java

@Entity
@Table(name = "groups")
public class MyGroup {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "groups_id")
    private Integer gropuId;

    @Column(name ="groupName")
    private String groupName;

    @ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
    @JoinTable(name = "friend_group", joinColumns = {
            @JoinColumn(name = "fk_group_id", referencedColumnName = "groups_id") }, inverseJoinColumns = {
                    @JoinColumn(name = "fk_friend_id", referencedColumnName = "friends_id") })
    private Set<Friend> friends = new HashSet<Friend>();

  //.... constructer , getter and setter

2. Friend.java

@Entity
@Table(name = "friends")
public class Friend {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "friends_id")
    private Integer friendId;

    @Column(name = "friendName")
    private String friendName;

    @ManyToMany(mappedBy = "friends")
    private Set<MyGroup> groups = new HashSet<MyGroup>();
  //.... constructer , getter and setter

範例資料

輸入一些範例資料

insert into friends (friendName) values ('Tom'),('Mary'),('Tina')
GO
insert into groups (groupName) values('high school'),('game'),('work')
GO

insert into friend_group (fk_friend_id, fk_group_id) values
(1,1),(1,2),(2,3),
(3,1),(3,2),(3,3)
GO

主要目標

假設 work 群組內原本有 Mary, Tina，今天有新來的同事 Bill 且 Tina 離職只剩 Mary, Bill 需要在工作群組。

動作: 先讓 work 群組只剩 Mary ，然後要把 Bill 加入朋友(Friend Table)，且加在 work 群組中。

也就是修改前如圖:

然後修改後表格應該要長這樣子:

以下為操作 Java 物件的作法

public class DemoManyToManyFriendEx2 {

    public static void main(String[] args) {

        try {

            SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
            Session session = sessionFactory.getCurrentSession();
            
            session.beginTransaction();

            MyGroup group3 = session.get(MyGroup.class, 3);
            
            System.out.println("group3.getGroupName()" + group3.getGroupName());

            Set<Friend> friends = group3.getFriends();
      
      // 因為有 Lazy Loading 的設置，必須先把 friends 初始化(initialize)，才會強制讀取多方物件。
            Hibernate.initialize(friends);
            System.out.println("----------------------------------");


            Iterator<Friend> it = friends.iterator();
            System.out.println("friends.iterator();");
            System.out.println("friends"+ friends);
      
      // 走訪每個 friends 裡面的成員，並只剩下 Mary
            while (it.hasNext()) {
                Friend friend = it.next();
                String name = friend.getFriendName();
                System.out.println("friend.getFriendName();" + name);

                if (!name.equals("Mary")) {
                    System.out.println("in if");
                    it.remove();
                }
            }
            
            System.out.println("friends.size();"+friends.size());
      

      // 新增 Bill 為朋友，且加入透過 work 群組拿到的 friends set
            Friend aNewFriend = new Friend("Bill");
            friends.add(aNewFriend);
            
            System.out.println("friends.size();"+friends.size());

            session.getTransaction().commit();

            System.out.println("Commit OK!!");

        } catch (Exception e) {
            System.out.println("Something wrong and Roll Back!!");
            e.printStackTrace();
        } finally {
            HibernateUtil.closeSessionFactory();
        }

    }

}

成功的 console

session.beginTransaction();
Hibernate: 
    select
        mygroup0_.groups_id as groups_i1_8_0_,
        mygroup0_.groupName as groupnam2_8_0_ 
    from
        groups mygroup0_ 
    where
        mygroup0_.groups_id=?
group3.getGroupName()work
Hibernate: 
    select
        friends0_.fk_group_id as fk_group1_6_0_,
        friends0_.fk_friend_id as fk_frien2_6_0_,
        friend1_.friends_id as friends_1_7_1_,
        friend1_.friendName as friendna2_7_1_ 
    from
        friend_group friends0_ 
    inner join
        friends friend1_ 
            on friends0_.fk_friend_id=friend1_.friends_id 
    where
        friends0_.fk_group_id=?
----------------------------------
friends.iterator();
friends[Friend [friendId=2, friendName=Mary], Friend [friendId=3, friendName=Tina]]
friend.getFriendName();Mary
friend.getFriendName();Tina
in if
friends.size();1
friends.size();2
Hibernate: 
    insert 
    into
        friends
        (friendName) 
    values
        (?)
Hibernate: 
    delete 
    from
        friend_group 
    where
        fk_group_id=? 
        and fk_friend_id=?
Hibernate: 
    insert 
    into
        friend_group
        (fk_group_id, fk_friend_id) 
    values
        (?, ?)
Commit OK!!

其他作法

1. 也可以使用 HQL 方法。
2. 使用 Hibernate 的 createSQLQuery 物件操作原生的 SQL 語法。