hibernate（九）多对多关联

2022-10-14 08:58:58

原文链接：http://www.orlion.ml/29/

一、多对多单向关联

假设一个老师教多个学生，一个学生被多个老师教，这就是典型的多对多关系

配置方式是在Teacher类的getStudents()方法上添加注解@ManyToMany

@JoinTable(name="t_s" ,

joinColumns = { @JoinColumn(name="teacher_id")},

inverseJoinColumns = { @JoinColumn(name="student_id")}

)

Teacher类:

package cn.orlion.hibernate.model;

import java.util.HashSet;

import java.util.Set;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.Id;

import javax.persistence.JoinColumn;

import javax.persistence.JoinTable;

import javax.persistence.ManyToMany;

@Entity

public class Teacher {

    private int id;

    private String name;

    private Set<Student> students = new HashSet<Student>();

    @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;

    }

    @ManyToMany

    @JoinTable(name="t_s" , // t_s是生成的中间表的表名

        joinColumns = { @JoinColumn(name="teacher_id")},// teacher_id是生成中间表中参考自Teacher的外键

        inverseJoinColumns = { @JoinColumn(name="student_id")}// inverseJoinColumns配置的是反转的对象（即Student）的idstudent_id是中间表中参考自Student的外键

        )

    public Set<Student> getStudents() {

        return students;

    }

    public void setStudents(Set<Student> students) {

        this.students = students;

    }

}

Student类：

package cn.orlion.hibernate.model;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.Id;

@Entity

public class Student {

    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语句：

create table Student (

        id integer not null auto_increment,

        name varchar(255),

        primary key (id)

    )

create table Teacher (

        id integer not null auto_increment,

        name varchar(255),

        primary key (id)

    )

create table t_s (

        teacher_id integer not null,

        student_id integer not null,

        primary key (teacher_id, student_id)

    )

alter table t_s

        add constraint FK_3no0wm9bwtogr87t4gvd4t3x5

        foreign key (student_id)

        references Student (id)

alter table t_s

        add constraint FK_pj2fmd6skwfi7jm2avlo4n6mf

        foreign key (teacher_id)

        references Teacher (id)

二、多对多双向关联

单向关联可一理解为：老师知道教的学生的名字，但是学生不知道教的多个老师。

双向关联可以理解为：老师知道教的学生的名字，学生也知道教的多个老师。

配置方式：Teacher类与单向关联相同，Student类添加Set<Teacher> teachers属性，在getTeaches()方法上添加注解@ManyToMany(mappedBy="students")

Teacher类：

package cn.orlion.hibernate.model;

import java.util.HashSet;

import java.util.Set;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.Id;

import javax.persistence.JoinColumn;

import javax.persistence.JoinTable;

import javax.persistence.ManyToMany;

@Entity

public class Teacher {

    private int id;

    private String name;

    private Set<Student> students = new HashSet<Student>();

    @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;

    }

    @ManyToMany

    @JoinTable(name="t_s" , // t_s是生成的中间表的表名

        joinColumns = { @JoinColumn(name="teacher_id")},

        inverseJoinColumns = { @JoinColumn(name="student_id")}

        )

    public Set<Student> getStudents() {

        return students;

    }

    public void setStudents(Set<Student> students) {

        this.students = students;

    }

}

Student类：

package cn.orlion.hibernate.model;

import java.util.HashSet;

import java.util.Set;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.Id;

import javax.persistence.ManyToMany;

@Entity

public class Student {

    private int id;

    private String name;

    private Set<Teacher> teachers = new HashSet<Teacher>();

    @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;

    }

    @ManyToMany(mappedBy="students")

    public Set<Teacher> getTeachers() {

        return teachers;

    }

    public void setTeachers(Set<Teacher> teachers) {

        this.teachers = teachers;

    }

}

生成的sql语句：

create table Student (

        id integer not null auto_increment,

        name varchar(255),

        primary key (id)

    )

create table Teacher (

        id integer not null auto_increment,

        name varchar(255),

        primary key (id)

    )

create table t_s (

        teacher_id integer not null,

        student_id integer not null,

        primary key (teacher_id, student_id)

    )

alter table t_s

        add constraint FK_3no0wm9bwtogr87t4gvd4t3x5

        foreign key (student_id)

        references Student (id)

alter table t_s

        add constraint FK_pj2fmd6skwfi7jm2avlo4n6mf

        foreign key (teacher_id)

        references Teacher (id)

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