hibernate(九)多对多关联
2024-08-31 20:44:46
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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