多对多manytomany

----------------------多对多manytomany
要点：
多对多必须有mappedBy，让一个端维护关系
多对多会创建一张关系表，维护关系就是维护第三张表。
所以增加关系，则在集合中添加实体。删除关系，则从集合中删除实体。

mappedBy使一方丧失关系维护

@Entity
public class User {

	@Id
	@GeneratedValue
	private long id;

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

	@ManyToMany(mappedBy="users")
	@Cascade(value=CascadeType.SAVE_UPDATE)
	private Set<Role> roles = new HashSet<Role>();
}

@Entity
public class Role {
	@Id
	@GeneratedValue
	private long id;
	@Column
	private String name;
	@ManyToMany
	private Set<User> users = new HashSet<User>();
	public String getName() {
		return name;
} 

--------------------------------保存mappedBy端
		User user = new User();
        user.setName("zhangsan");
        User user2 = new User();
        user2.setName("lisi");

        Role r1= new Role();
        r1.setName("ADMIN");
        Role r2= new Role();
        r2.setName("EDITOR");

        user.getRoles().add(r1);
        user.getRoles().add(r2);
        user2.getRoles().add(r1);

        r1.getUsers().add(user);
        r1.getUsers().add(user2);
        r2.getUsers().add(user);

        session.save(user);
        session.save(user2);

结果：

进行了级联保存。
如果使用

CascadeType.PERSIST注解

@ManyToMany(cascade=CascadeType.PERSIST,mappedBy="users") 则需要调用session.persist方法进行保存。注意每个方法上都有支持的级联注解的说明！

Hibernate:
    insert
    into
        User
        (name, id)
    values
        (?, ?)
Hibernate:
    insert
    into
        Role
        (name, id)
    values
        (?, ?)
Hibernate:
    insert
    into
        Role
        (name, id)
    values
        (?, ?)
Hibernate:
    insert
    into
        User
        (name, id)
    values
        (?, ?)
Hibernate:
    insert
    into
        Role_User
        (roles_id, users_id)
    values
        (?, ?)
Hibernate:
    insert
    into
        Role_User
        (roles_id, users_id)
    values
        (?, ?)
Hibernate:
    insert
    into
        Role_User
        (roles_id, users_id)
    values
        (?, ?)

删除用户139的ADMIN权限
		User u = session.get(User.class,139L);
        Set<Role> roles = u.getRoles();
        Iterator<Role> ites = roles.iterator();
        while(ites.hasNext()) {
			Role r = ites.next();
			if(r.getName().equals("ADMIN")){
				r.getUsers().remove(u);//执行成功，删除关系，而不删除用户
				//u.getRoles().remove(r);//执行失败，关系它不维护
			}
		}