Question

我创建了Rest Api，它将学生数据保存到数据库。而且学生编号不会成为主键，我不希望它是自动生成的，这就是为什么我没有给出任何生成策略的原因。

学生可以有很多笔，所以有One To Many Mapping。每当我尝试将数据保存到db时，都会出现错误 A different object with the same identifier value was already associated with the session.。

我知道每个主键在会话中只能有一个对象

DAO LAYER

    @Autowired
        private EntityManager entityManager;

    public void saveStudent(Students student) {
  // here i am saving data
            entityManager.persist(student);
        }

Pojo学生：

    @Entity
    @Data
    @AllArgsConstructor
    @Table(name = "list_of_students")
    @NoArgsConstructor
    public class Students implements Serializable {

        private static final long serialVersionUID = -163504605143278308L;

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

        @Id
        @Column(name = "student_roll_no")
        private Long rollNo;

        @OneToMany(fetch = FetchType.EAGER, mappedBy = "student", cascade = {CascadeType.ALL})
        private Set<Pen> pens = new HashSet<Pen>();

        @ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
        @JoinTable(name = "student_courses", joinColumns = { @JoinColumn(name = "student_roll_no") }, inverseJoinColumns = {
                @JoinColumn(name = "course_id") })
        private Set<Courses> listOfCourses = new HashSet<Courses>();

    }

Pen Pojo：

@Entity
@NoArgsConstructor
@AllArgsConstructor
@Data
@Table(name = "pens")
public class Pen {

    @Id
    @Column(name = "pen_color",unique=true,columnDefinition="VARCHAR(64)")
    private String color;

    @ManyToOne(fetch = FetchType.EAGER,cascade =CascadeType.ALL)
    @JoinColumn(name="student_roll_no")
    private Students student;
}

我尝试执行以下提到的操作：通过这样做，我没有收到任何错误，但没有数据保存到pens Table 在“笔”表中，我希望学生卷不映射到笔的颜色。

@OneToMany(fetch = FetchType.EAGER, mappedBy = "student", cascade = {CascadeType.MERGE})
    private Set<Pen> pens = new HashSet<Pen>();

REST CONTROLLER

@ApiOperation(value = "Add Student")
@RequestMapping(value = "add/Student", method = RequestMethod.POST, consumes = MediaType.APPLICATION_JSON_UTF8_VALUE, produces = MediaType.APPLICATION_JSON_UTF8_VALUE)
public void addStudentData(@Valid @RequestBody Students student) {

    service.save(student);

}

邮递员请求

URL：http://localhost:8080/add/Student

{
    "name": "Ashish Upadhyay",
    "rollNo": "1",
    "pens": [{
        "color": "RED",
        "student":{
            "rollNo":"1"
        }
    }],
    "listOfCourses": [{
            "courseName": "Data Structures",
            "courseId": 1
        },
        {
            "courseName": "Alogrithms",
            "courseId": 2
        },
        {
            "courseName": "Pysical Education",
            "courseId": 3
        }
    ]
}

具有相同标识符值的另一个对象已与会话休眠相关联

0 个答案: