在 Hibernate 双向 ManytoOne、OnetoMany 的映射列中获取 null

Posted 2023-02-26

【中文标题】在 Hibernate 双向 ManytoOne、OnetoMany 的映射列中获取 null

【英文标题】：Getting null in mapping column in Hibernate bidirectional ManytoOne , OnetoMany

【发布时间】：2019-05-22 21:54:03

【问题描述】：

我正在尝试创建双向 OnetoMany、ManytoOne 映射。下面是实体类 StudentAddress 和 Student3 类：

Student3实体类：

@Entity
@Table(name="Student3", schema="cf_2583f365_c3c6_499a_a60d_138e7e7023eb")
public class Student3 
@Id
@Column(name = "STUDENT_ID")
@GeneratedValue(strategy=GenerationType.IDENTITY)
private int student_id;

@Column(name = "STUDENT_NAME")
private String student_name;

@ManyToOne(cascade=CascadeType.ALL)
@JoinColumn(name="student_address_id",referencedColumnName="address_id")
/*@JoinColumn(name = "student_address_id")*/
private StudentAddress studentAddress;


@Override
public String toString() 
    return "Student3 [student_id=" + student_id + ", STUDENT_NAME=" + student_name + "]";


public int getStudent_id() 
    return student_id;


public void setStudent_id(int student_id) 
    this.student_id = student_id;


public String getStudent_name() 
    return student_name;


public void setStudent_name(String student_name) 
    this.student_name = student_name;


public StudentAddress getStudentAddress() 
    return studentAddress;


public void setStudentAddress(StudentAddress studentAddress) 
    this.studentAddress = studentAddress;


public Student3() 

学生地址实体类：

@Entity
@Table(name="StudentAddress", schema="cf_2583f365_c3c6_499a_a60d_138e7e7023eb")
public class StudentAddress 

@Id
@Column(name = "address_id")
@GeneratedValue(strategy=GenerationType.IDENTITY)
private int address_id;

@Column(name = "address_detail")
private String address_detail;

@OneToMany(cascade=CascadeType.ALL,mappedBy = "studentAddress")
private List<Student3> students;



@Override
public String toString() 
    return "StudentAddress [address_id=" + address_id + ", address_detail=" + address_detail + ", students="
            + students + "]";


public int getAddress_id() 
    return address_id;


public void setAddress_id(int address_id) 
    this.address_id = address_id;


public String getAddress_detail() 
    return address_detail;


public void setAddress_detail(String address_detail) 
    this.address_detail = address_detail;


public List<Student3> getStudents() 
    return students;


public void setStudents(List<Student3> students) 
    this.students = students;



public StudentAddress() 

在保存 Student3 对象的数据时，我可以像这样保存数据：

但是当我从 StudentAddress 端保存数据时，我在 student_address_id 列中得到一个空值：

从 Student3，studentAddress 端保存数据的代码：

/**************SAVE STUDENT Address Controller*************************/
@RequestMapping(value="/saveStudentAddress", method=RequestMethod.POST,consumes=MediaType.APPLICATION_JSON_VALUE)
public String savestudentaddress (@RequestBody StudentAddress s1) 

    System.out.println(s1);
    studentaddressRepository.save(s1);
    return "success";


/**************SAVE STUDENT3 Controller*************************/
@RequestMapping(value="/saveStudent3", method=RequestMethod.POST,consumes=MediaType.APPLICATION_JSON_VALUE)
public String savestudent (@RequestBody Student3 s1) 

    System.out.println(s1);
    student3Repository.save(s1);
    return "success";

从 Student3 端保存数据的 JSON：

    "student_id" :123, 
    "student_name": "avijit kumar Mamod",

    "studentAddress":
                    "address_id" : 123,
                    "address_detail" : "209 - Berlington Street"
                               

从 StudentAddress 端保存数据的 Json：

"address_id" : 123,
"address_detail" : "209 - Berlington Street",
"students": [
            "student_id" :123, 
            "student_name": "avijit kumar Mamod"
           ]

【问题讨论】：

您能否展示一下您如何从 StudentAddress 端保存数据的代码？

@Abhi 我已经更新了我的 qn ，添加了相同的代码。

【参考方案1】：

今天，从StudentAddress 一侧删除mappedBy 并放入@JoinColumn(name="student_address_id")，当我尝试从studentAddress 一侧保存时，它确实在DB student_address_id 中增加了价值。

studentAddress 现在看起来像这样：

@OneToMany(cascade=CascadeType.ALL/*,mappedBy = "studentAddress"*/)
@JoinColumn(name="student_address_id")
private List<Student3> students;

我不知道为什么会这样。我没有上面答案中abhi建议的额外代码。控制器现在很简单：

@RequestMapping(value="/saveStudentAddress", method=RequestMethod.POST,consumes=MediaType.APPLICATION_JSON_VALUE)
public String savestudentaddress (@RequestBody StudentAddress s1) 
    System.out.println(s1);
   studentaddressRepository.save(s1);
    return "success";

【参考方案2】：

修改您的 saveStudentAddress 代码，如下所示并尝试

@RequestMapping(value="/saveStudentAddress", method=RequestMethod.POST,consumes=MediaType.APPLICATION_JSON_VALUE)
public String savestudentaddress (@RequestBody StudentAddress s1) 
    System.out.println(s1);

    for(Student s: s1.getStudents())
      s.setAddress(s1);
    

    studentaddressRepository.save(s1);
    return "success";

【讨论】：

它的工作！但我不明白，我们甚至没有保存's'，我们还在保存's1'。为什么设置 s, wud 会有这么大的不同？？

这就是 Hibernate 文档在这种情况下建议保存对象的方式。希望它有所帮助。

请看下面我的回答，这更离奇

【参考方案3】：

不要在两个类中都写 OneToMany。

【讨论】：

我已经编辑了我的问题，我错误地添加了 StudentAddress 类两次。现在Student3课已经上线了。