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)