hibernate（八）一对多关联

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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)