Spring boot入门之-Jpa- One to Many Many to one

代码参考git。git地址：https://github.com/lidreamwind/Java-Jpa-Data

one to many是一张表的一条记录对应另一张表的多条记录。

Many to one 是一张表的多条记录对应另一张表的一条记录。

两张表之间以外键关系关联在一起。

文档参考：https://download.oracle.com/otn-pub/jcp/persistence-2_1-fr-eval-spec/JavaPersistence.pdf?AuthParam=1574061512_630dd51b88a4d03476be822a6903bad5

1、OneToMany配置如下　　

package com.lph.spring.dev.ManyToOne.pojo;

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

@Entity
@Table(name = "student")
public class StudentEntity {
    @Id
    @GeneratedValue
    @Column(name =  "student_id")
    private Integer studentId;
    @Column(name = "name")
    private String name;
    @Column(name = "age")
    private Integer age;

    //studentId是实体类中的属性，mappedBy不能和@JoinTable一起
    @OneToMany(targetEntity = CourseStudent.class,mappedBy = "studentId",fetch = FetchType.EAGER,cascade = CascadeType.ALL)
    private List<CourseStudent> courseStudent ;

    public Integer getStudentId() {
        return studentId;
    }

    public void setStudentId(Integer studentId) {
        this.studentId = studentId;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public List<CourseStudent> getCourseStudent() {
        return courseStudent;
    }

    public void setCourseStudent(List<CourseStudent> courseStudent) {
        this.courseStudent = courseStudent;
    }
}

2、Many To One的配置如下　　

package com.lph.spring.dev.ManyToOne.pojo;

import com.fasterxml.jackson.annotation.JsonIgnore;

import javax.persistence.*;

@Entity
@Table(name = "couurse_student")
public class CourseStudent {
    @Id
    @GeneratedValue
    @Column(name = "id")
    private Integer id;
    @Column(name = "course_id")
    private Integer courseId;
    @Column(name = "student_id")
    private Integer studentId;

    @ManyToOne()
    @JoinColumn(name = "student_id",insertable = false,updatable = false,referencedColumnName = "student_id")
    @JsonIgnore
    private StudentEntity studentEntity;

    public Integer getId() {
        return id;
    }

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

    public Integer getCourseId() {
        return courseId;
    }

    public void setCourseId(Integer courseId) {
        this.courseId = courseId;
    }

    public Integer getStudentId() {
        return studentId;
    }

    public void setStudentId(Integer studentId) {
        this.studentId = studentId;
    }

    public StudentEntity getStudentEntity() {
        return studentEntity;
    }

    public void setStudentEntity(StudentEntity studentEntity) {
        this.studentEntity = studentEntity;
    }
}

3、One To Many的配置解析。　　

@OneToMany(targetEntity = CourseStudent.class,mappedBy = "studentId",fetch = FetchType.EAGER,cascade = CascadeType.ALL) 
private List<CourseStudent> courseStudent ;

OneToMany的有两行，一行注解和一个List变量。List变量用来存放一对多的结果。
targetEntity的是用来注释需要和哪个实体类进行联合查询(实体类对应表),mappedBy指定的是实体类中的链接字段，对应表中的字段。
fetch有两个类型，LAZY和EAGER　　
　　参考：https://stackoverflow.com/questions/26601032/default-fetch-type-for-one-to-one-many-to-one-and-one-to-many-in-hibernate
cascade有ALL，DETACH,MERGE,PERSIST,REFRESH,REMOVE几种类型。　　
　　参考：https://vladmihalcea.com/a-beginners-guide-to-jpa-and-hibernate-cascade-types/
官网文档 ，mappedBy不能用于ManyToOnne注解。
The many side of one-to-many / many-to-one bidirectional relationships must be the owning
side, hence the mappedBy element cannot be specified on the ManyToOne annotation.

4、Many To One的配置解析。　　

@ManyToOne() 
@JoinColumn(name = "student_id",insertable = false,updatable = false,referencedColumnName = "student_id") 
@JsonIgnore 
private StudentEntity studentEntity;
@ManyToOne是标注为多对一
@JoinColumn是标注匹配的外键列，insertable = false，不允许插入，updatable=false是不允许更新。referenceColumnName标注的是外键列。
@JsonIgnore，生成Json时不生成StudentEntity 的属性
5、结果展示