hibernate注解（二）mappedBy

1.双向关系中使用mappedBy避免生成中间表

(1).只有OneToOne，OneToMany，ManyToMany上才有mappedBy属性，ManyToOne不存在该属性；

(2).mappedBy标签一定是定义在被拥有方(Group)，他指向拥有方(User)；

@Entity
@Table(name="t_group")
public class Group {
    private int id;
    private String name;
    private Set<User> users = new HashSet<User>();
    public Group(){
        
    }
    public Group(String name) {
        this.name = name;
    }
    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @OneToMany
    public Set<User> getUsers() {
        return users;
    }
    public void setUsers(Set<User> users) {
        this.users = users;
    }
}

@Entity
@Table(name="t_user")
public class User {
    private int id;
    private String name;
    private Group group;
    
    public User(){
        
    }
    public User(String name) {
        this.name = name;
    }
    @ManyToOne
    @JoinColumn(name="mygroudid")
    public Group getGroup() {
        return group;
    }
    public void setGroup(Group group) {
        this.group = group;
    }
    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

2.会生成sql语句

create table t_group (id integer not null auto_increment, name varchar(255), primary key (id))
 create table t_group_t_user (

t_group_id integer not null,

users_id integer not null,

primary key (t_group_id, users_id),

unique (users_id)

)
create table t_user (id integer not null auto_increment, name varchar(255), mygroudid integer, primary key (id))

alter table t_group_t_user add index FKCFE61C614F1D14B3 (t_group_id), add constraint FKCFE61C614F1D14B3 foreign key (t_group_id) references t_group (id)
 alter table t_group_t_user add index FKCFE61C61363448CF (users_id), add constraint FKCFE61C61363448CF foreign key (users_id) references t_user (id)
alter table t_user add index FKCB63CCB6C650D60F (mygroudid), add constraint FKCB63CCB6C650D60F foreign key (mygroudid) references t_group (id)
3.如果在在Group中

    @OneToMany(mappedBy="group")      //“group”对应User类中的group属性。不能写错
    public Set<User> getUsers() {
        return users;
    }
    public void setUsers(Set<User> users) {
        this.users = users;
    }

create table t_group (id integer not null auto_increment, name varchar(255), primary key (id))
create table t_user (id integer not null auto_increment, name varchar(255), mygroudid integer, primary key (id))
alter table t_user add index FKCB63CCB6C650D60F (mygroudid), add constraint FKCB63CCB6C650D60F foreign key (mygroudid) references t_group (id)