hibernate（八）一对多关联

一、一对多单向关良

一对多单向关联与多对一相似

假设一个组有多个用户，即一(Group)对多(User)

需要在Group类中添加一个User类的Set集合(数据库中的用户不可能是重复的，所以要用Set)属性:private Set<User> users;然后在getUser()方法上添加注解：
@OneToMany和@JoinColumn(name="groupId"),如果不加@JoinColumn则会自动生成一个中间表

Group类：

package cn.orlion.hibernate.model;

import java.util.Set;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Entity
@Table(name="t_group")
public class Group {

private int id;

private String name;

private Set<User> users;
@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
@JoinColumn(name="groupId")
public Set<User> getUsers() {
return users;
}

public void setUsers(Set<User> users) {
this.users = users;
}
}

User类：

package cn.orlion.hibernate.model;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name="t_user")
public class User {

private int id;

private String 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;
}
}

生成的sql：

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),
groupId integer,
primary key (id)
)

alter table t_user
add constraint FK_7ktm6l2qkykpqrf6oq01ys8wy
foreign key (groupId)
references t_group (id)

二、一对多双向关联

配置方式：在Group类中的getUsers()方法上添加注解@OneToMany(mappedBy="group"),如果不写mappedBy则会在user表中生成两个groupid字段

；然后在User类中添加group属性，在getGroup()方法上添加注解@ManyToOne

Group类：

package cn.orlion.hibernate.model;

import java.util.Set;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Entity
@Table(name="t_group")
public class Group {

private int id;

private String name;

private Set<User> users;
@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(mappedBy="group")
public Set<User> getUsers() {
return users;
}

public void setUsers(Set<User> users) {
this.users = users;
}
}

User类：

package cn.orlion.hibernate.model;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

@Entity
@Table(name="t_user")
public class User {

private int id;

private String name;

private 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;
}
@ManyToOne
public Group getGroup() {
return group;
}

public void setGroup(Group group) {
this.group = group;
}
}

生成的sql

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),
group_id integer,
primary key (id)
)

alter table t_user
add constraint FK_e5f24mh6aryt9hsy99oydps6g
foreign key (group_id)
references t_group (id)