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JPA中的@ManyToMany

@ManyToMany註釋表示模型類是多對多關係的一端。

@JoinTable 描述了多對多關係的資料表關係。

• name 屬性指定中間表名稱
• joinColumns 定義中間表與Teacher 表的外來鍵關係
• inverseJoinColumns屬性定義了中間表與另外一端(Student)的外來鍵關係

單向多對多表的ddl語句

CREATE TABLE `t_student` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `sname` varchar(255) DEFAULT
 
 NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;

CREATE TABLE `t_teacher` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `tname` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

CREATE TABLE `t_teacher_student` (
  `teacher_id`
 
 bigint(20) NOT NULL,
  `student_id` bigint(20) NOT NULL,
  PRIMARY KEY (`teacher_id`,`student_id`),
  KEY `FK_o82lxg0wi1y88lit4fy7lkg7t` (`student_id`),
  CONSTRAINT `FK_1epgkih044inndyv1dtihcb7r` FOREIGN KEY (`teacher_id`) REFERENCES `t_teacher` (`id`),
  CONSTRAINT `FK_o82lxg0wi1y88lit4fy7lkg7t` FOREIGN
 
 KEY (`student_id`) REFERENCES `t_student` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Student

package com.jege.jpa.many2many;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

/**
 * @author JE哥
 * @email [email protected]
 * @description:單向
 */
@Entity
@Table(name = "t_student")
public class Student {
  @Id
  @GeneratedValue
  private Long id;
  private String sname;

  public Student() {

  }

  public Student(String sname) {
    this.sname = sname;
  }

  public Long getId() {
    return id;
  }

  public void setId(Long id) {
    this.id = id;
  }

  public String getSname() {
    return sname;
  }

  public void setSname(String sname) {
    this.sname = sname;
  }

  @Override
  public String toString() {
    return "Student [id=" + id + ", sname=" + sname + "]";
  }
}

Teacher

package com.jege.jpa.many2many;

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

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

/**
 * @author JE哥
 * @email [email protected]
 * @description:單向:一個老師教多個學生
 */
@Entity
@Table(name = "t_teacher")
public class Teacher {
  @Id
  @GeneratedValue
  private Long id;
  private String tname;
  // @ManyToMany註釋表示Teacher是多對多關係的一端。
  // @JoinTable描述了多對多關係的資料表關係。name屬性指定中間表名稱，joinColumns定義中間表與Teacher表的外來鍵關係。
  // 中間表Teacher_Student的Teacher_ID列是Teacher表的主鍵列對應的外來鍵列，inverseJoinColumns屬性定義了中間表與另外一端(Student)的外來鍵關係。
  @ManyToMany(cascade = CascadeType.PERSIST, fetch = FetchType.LAZY)
  @JoinTable(name = "t_teacher_student", joinColumns = { @JoinColumn(name = "teacher_id") }, inverseJoinColumns = {
      @JoinColumn(name = "student_id") })
  private Set<Student> students = new HashSet<Student>();

  public Teacher() {

  }

  public Teacher(String tname) {
    this.tname = tname;
  }

  public Long getId() {
    return id;
  }

  public void setId(Long id) {
    this.id = id;
  }

  public String getTname() {
    return tname;
  }

  public void setTname(String tname) {
    this.tname = tname;
  }

  public Set<Student> getStudents() {
    return students;
  }

  public void setStudents(Set<Student> students) {
    this.students = students;
  }

  @Override
  public String toString() {
    return "Teacher [id=" + id + ", tname=" + tname + "]";
  }
}

Many2ManyTest

package com.jege.jpa.many2many;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

import org.junit.After;
import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;

/**
 * @author JE哥
 * @email [email protected]
 * @description:單向多對多Test
 */
public class Many2ManyTest {
  private static EntityManagerFactory entityManagerFactory = null;
  private EntityManager entityManager = null;

  @BeforeClass
  public static void setUpBeforeClass() throws Exception {
    entityManagerFactory = Persistence.createEntityManagerFactory("com.jege.jpa");
  }

  // t1老師教2個學生s1,s2
  // t2老師教3個學生s1,s2,s3
  // 總共10條insert
  @Test
  public void persist() throws Exception {
    entityManager = entityManagerFactory.createEntityManager();
    entityManager.getTransaction().begin();

    Teacher t1 = new Teacher("t1");
    Teacher t2 = new Teacher("t2");

    Student s1 = new Student("s1");
    Student s2 = new Student("s2");
    Student s3 = new Student("s3");

    entityManager.persist(t1);
    entityManager.persist(t2);

    entityManager.persist(s1);
    entityManager.persist(s2);
    entityManager.persist(s3);// 全部發出5條insert單表

    // 新增中間表
    // t1老師教2個學生s1,s2
    // t2老師教3個學生s1,s2,s3
    t1.getStudents().add(s1);
    t1.getStudents().add(s2);

    t2.getStudents().add(s1);
    t2.getStudents().add(s2);
    t2.getStudents().add(s3);

    entityManager.getTransaction().commit();// 發出5條insert中間表
    entityManager.close();
  }

  // t1老師教2個學生s1,s2
  // 修改為 教2個學生s1,s3:先刪除在新增
  @Test
  public void update() throws Exception {
    persist();

    entityManager = entityManagerFactory.createEntityManager();
    entityManager.getTransaction().begin();

    Teacher t1 = entityManager.find(Teacher.class, 1L);
    Student s2 = entityManager.find(Student.class, 2L);
    Student s3 = entityManager.find(Student.class, 3L);
    // 刪除
    t1.getStudents().remove(s2);
    // 新增
    t1.getStudents().add(s3);

    entityManager.getTransaction().commit();
  }

  // 刪除t1的教的所有學生
  @Test
  public void delete() throws Exception {
    persist();

    entityManager = entityManagerFactory.createEntityManager();
    entityManager.getTransaction().begin();

    Teacher t1 = entityManager.find(Teacher.class, 1L);
    t1.getStudents().clear();

    entityManager.getTransaction().commit();
  }

  @After
  public void tearDown() throws Exception {
    if (entityManager != null && entityManager.isOpen())
      entityManager.close();
  }

  @AfterClass
  public static void tearDownAfterClass() throws Exception {
    if (entityManagerFactory != null && entityManagerFactory.isOpen())
      entityManagerFactory.close();
  }
}

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