多对一关系不会填充新实体

Question

My goal was to create a new entity to handle the problem with @ManyToMany relationship. I'm wondering to audit new entity but hibernate does not populate the CourseRegistration-new entity with data.

@Entity
@Table(name = "course_registration")
public class CourseRegistration {

    @Id
    @Column(name = "id")
    private Long id;

    @ManyToOne
    @JoinColumn(name = "student_id")
    private Student student;

    @ManyToOne
    @JoinColumn(name = "course_id")
    private Course course;

    public CourseRegistration() {}
}


@Entity
@Table(name = "student")
public class Student {

    @Id
    @Column(name = "id")
    private Long id;

    @OneToMany(mappedBy = "student")
    private List<CourseRegistration> registrations = new ArrayList<>();

    public Student() {}
}

@Entity
@Table(name = "course")
public class Course {

    @Id
    @Column(name = "id")
    private Long id;

    @OneToMany(mappedBy = "course")
    private List<CourseRegistration> registrations = new ArrayList<>();

    public Course() {}
}

Answer 1

You have to mention fetch type in your mappings and also add getter and setter if not added to your entity like below.

@Entity
@Table(name = "course_registration")
@Data
@NoArgsConstructor
@AllArgsConstructor
public class CourseRegistration {

    @Id
    @Column(name = "id")
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
    @JsonIgnore
    @JoinColumn(name = "student_id")
    private Student student;

    @ManyToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
    @JsonIgnore
    @JoinColumn(name = "course_id")
    private Course course;

}


@Entity
@Table(name = "student")
@Data
@NoArgsConstructor
@AllArgsConstructor
public class Student {

    @Id
    @Column(name = "id")
    private Long id;

    @OneToMany(mappedBy = "student")
    private List<CourseRegistration> registrations = new ArrayList<>();
    
}

@Entity
@Table(name = "course")
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Course {

    @Id
    @Column(name = "id")
    private Long id;

    @OneToMany(mappedBy = "course")
    private List<CourseRegistration> registrations = new ArrayList<>();
    
}

add below dependency to enable getter and setter with lambok.

<dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>

Answer 2

My bet is that you are missing the cascade option on your @OneToMany annotation. Try the following:

@Entity
@Table(name = "course")
public class Course {

    @Id
    @Column(name = "id")
    private Long id;

    @OneToMany(mappedBy = "course", cascade = CascadeType.ALL)
    private List<CourseRegistration> registrations = new ArrayList<>();

    public Course() {}
}

ALL means that the following operation are propagaed to the associated entity (in this case CourseRegistration ): PERSIST, MERGE, REMOVE, REFRESH and DETACH.