Question

我创建了两个表格-学生和主题。然后，我创建了第三个表student_subject，这是一个将学生与科目联系起来的联合表。它有5个字段：行ID，学生ID，学科ID和created_at和Updated_at字段（来自AuditModel类）：

  package com.rest.web.postgresapi.data.models;

import com.fasterxml.jackson.annotation.JsonIgnore;
import javax.persistence.*;
import org.hibernate.annotations.OnDelete;
import org.hibernate.annotations.OnDeleteAction;

@Entity
@Table(uniqueConstraints = {
        @UniqueConstraint(columnNames = {"student_id", "subject_id"})
})
public class StudentSubject extends AuditModel {

    @Id
    @GeneratedValue(generator = "enrollment_generator")
    @SequenceGenerator(
            name = "enrollment_generator",
            sequenceName = "enrollment_sequence",
            initialValue = 4420
    )
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "student_id", nullable = false)
    @OnDelete(action = OnDeleteAction.CASCADE)
    @JsonIgnore
    private Student student_id;  // If I put private Long student_id, it fails

    @ManyToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "subject_id", nullable = false)
    @OnDelete(action = OnDeleteAction.CASCADE)
    @JsonIgnore
    private Subject subject_id; // If I put private Long subject_id, it fails

    // Constructors
    protected StudentSubject() {}

    public StudentSubject(Student student_id, Subject subject_id) {
        this.student_id = student_id;
        this.subject_id = subject_id;
    }

    // Getters
    public Long getId() {
        return id;
    }

    public Student getStudent_id() {
        return student_id;
    }

    public Subject getSubject_id() {
        return subject_id;
    }

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

    public void setStudent_id(Student student) {
        this.student_id = student;
    }

    public void setSubject_id(Subject subject) {
        this.subject_id = subject;
    }

}

应用程序完美地在数据库中创建了表格，我可以在student和subject表格中进行获取和发布。没问题。疼痛来自关节台控制器。

这是student_subject联合表表的控制器

package com.rest.web.postgresapi.controllers;

import com.rest.web.postgresapi.data.models.StudentSubject;
import com.rest.web.postgresapi.repository.StudentSubjectRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

import javax.validation.Valid;

import java.util.List;

@RestController
public class StudentSubjectController {

    @Autowired
    private StudentSubjectRepository studentSubjectRepository;

    @GetMapping("/enrollments")
    public List<StudentSubject> getAllStudentsSubjects(){
        return studentSubjectRepository.findAll();
    }

    @PostMapping("/enrollments/student/subject")
    public StudentSubject createStudentSubject(@Valid @RequestBody StudentSubject studentSubject) {
        return studentSubjectRepository.save(studentSubject);
    }

}

有两个问题：

1 .-当我从student_subject表中进行获取时，它仅检索行的ID以及created_at和Updated_at字段。没有student_id或subject_id。

response from get

2 .--当我（从邮递员）张贴帖子以插入一行时，出现以下错误：

Detail: Failing row contains (4671, 2018-11-20 11:04:34.176, 2018-11-20 11:04:34.176, null, null).

我同时提供了Student_id和subject_id，如您在邮递员的此屏幕截图中所见，但错误明确指出两个字段均为空：

postman post

看来我对表格的定义有误。我的代码中缺少什么？

Answer 1

Spring MVC使用Jackson来将Java对象与JSON序列化/反序列化。

如果使用 @JSONIgnore 注释属性，那么Jackson将忽略它。

这就是为什么在GET方法的JSON响应中看不到 student_id 字段或 subject_id 字段的原因。因为当从对象转换为json时，杰克逊会忽略它们。

这就是您的POST失败的原因。因为Jackson忽略了接收到的属性。 Jackson正在创建一个空实体，而JPA正在保存没有 student_id 和 subject_id 的实体。

Answer 2

已通过替换

解决

@JsonIgnore

与

@JsonIgnoreProperties（{“ hibernateLazyInitializer”，“ handler”}）

如本answer

所示

Spring JPA联合表不会返回所有字段

2 个答案: