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《JPA ab ingressum ad proficiendiseries continet articulos sequentes:

• Iava pertinacia iacuit API: JPA
• Intellige JPA interface
• JPA modus interrogationis
• Articuli administrandi ratio (CRUD) ex JPA evoluta
• Necessitudo progressionem destinata (1): unum ad unum mapping
• Necessitudo progressionem destinata (2): unum ad multa destinata
• Relatio tabularum faciendarum (3): multi-ad-multi mapping

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exist multis multis In relatione consociationis tantum permedia mensa Mapping non potest effici claves exteras addendo.

annotation @ManyToMany Adhibetur enim mittens et receptor relationis. Finis relationis mittens definit agrum attributorum recipientis finis collectionis speciei, et acceptio finis relationis non indiget definitionibus aliqua facere.

1. crea entia

1,1 crea Discipulus ens

package com.example.demo.entity;

import lombok.Data;

import javax.persistence.*;
import java.util.List;
import java.util.Set;

@Entity
@Data
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;
    private String name;
    @Column(columnDefinition = "enum('male','female')")
    private String sex;

    @ManyToMany(fetch = FetchType.LAZY)
    @JoinTable(name = "teacher_student", joinColumns = {@JoinColumn(name = "s_id")}, inverseJoinColumns = {@JoinColumn(name = "t_id")})
    private Set<Teacher> teachers;
}
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1.2 crea Magister ens

package com.example.demo.entity;

import lombok.Data;

import javax.persistence.*;
import java.util.List;
import java.util.Set;

@Data
@Entity
public class Teacher {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;
    private String name;
    @ManyToMany(fetch = FetchType.LAZY)
    /**
     * Description:
     * 1、关系两边都作为主控；
     * 2、joinColumns 中 @JoinColumn(name="t_id") 其中 t_id 为 JoinTable 中的外键，由于 Student 和 Teacher 的主键都为 id 这边就省略 referencedColumnName="id"。
     */
    @JoinTable(name = "teacher_student", joinColumns = {@JoinColumn(name = "t_id")}, inverseJoinColumns = {@JoinColumn(name = "s_id")})
    private Set<Student> students;
}
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exist multis multis Hic notanda sunt quaedam in relatione tua:

• Utraque pars ad relationem domini servire potest.
• exist joinColumns of *@JoinColumn(name="t_id") medium,t_id Estne clavis extera in JoinTable.Cum claves primariae Discipuli et Magistri ambo suntid, ita hic omittitur referencedColumnName="id"。
• Cum inter exempla relatio cascades designet, considerare necesse est quaenam cascades regulae adhibeantur.
• Si paro cascade = CascadeType.PERSISTErgo exequens save vocabituronPersist() modum.Hoc modum vocamus recursively genus externum (Discipulus vel Magister)onPersist() Ad- praestare vauiis.Sed quia valor additus est, diceturdetached entity passed to persist Error, excludit operationem Caesar (removecascade = CascadeType.PERSIST).

2. crea probat

Servitium ac repositoria in stratis "Necessitudo progressionem destinata (1): unum ad unum mapping》 Iam memoravi, nihil interest hic, ut in details non ingrediar et directe ad codicem e tabula testi discursionis accedat.

package com.example.demo.entity;

import com.example.demo.repository.StudentRepository;
import com.example.demo.repository.TeacherRepository;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

import java.util.HashSet;
import java.util.Set;

import static org.junit.Assert.*;

@SpringBootTest
@RunWith(SpringRunner.class)
public class ManyToManyTest {
    @Autowired
    private StudentRepository studentRepository;
    @Autowired
    private TeacherRepository teacherRepository;

    @Test
    public void add() {

        Set<Teacher> teachers = new HashSet<>();
        Set<Student> students = new HashSet<>();

        Student student1 = new Student();
        student1.setName("张三");
        students.add(student1);
        studentRepository.save(student1);

        Student student2 = new Student();
        student2.setName("李四");
        students.add(student2);
        studentRepository.save(student2);

        Teacher teacher1 = new Teacher();
        teacher1.setName("皮皮老师");
        teacher1.setStudents(students);
        teachers.add(teacher1);
        teacherRepository.save(teacher1);
    }
}
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Currere in test genus incontrollerProventus sequentes sunt output:

🚀 对于双向 ManyToMany 关系，注解 @ManyToMany 用于关系的发出端和接收端。另外，关系的接收端需要设置 @ManyToMany(mappedBy='集合类型发出端实体的字段名称')。