OneToManyToMany Successively three tables with Hibernate Annotations

Question

I need to connect successively three tables with Hibernate Annotations.The second table should be dependent of the first. Then the third table should be dependent on the first and second tables. And in the result I should have some Java Map for parsing it into the json like this.

    {
      "id": 203,
      "idGroup": 3,
      ......
      "subjects": [
        {
          "id": 121,
          "groups": 3,
          .....
          "studentsModel": null
          "marks": [
            {
              "id": 1,
              "id_person": 203,
              "id_subject": 121,
              "mark": 5
            }
          ]
        },
        {
          ........
        }
      ]
    }

I think SQL will be like this, but I don't know how to do it with hibernate, but this example is not in inconsistent manner, like Json

SELECT * FROM students AS t1
INNER JOIN subjects t2 ON t2.groups = t1.id_group
INNER JOIN marks t3 ON (t3.id_subject=t2.id AND t3.id_stud=t1.id)

Here there is an image of this connecting:

Schema of tables

I have Hibernate classes:

Students entity:

            @Entity
            @Table(name = "students")
            public class StudentsModel implements Externalizable {
                @Id
                @Column(name = "id")
                @GeneratedValue(strategy = GenerationType.IDENTITY)
                private int id;               
                @Column(name = "id_group")
                private Integer idGroup;
                .......
                @OneToMany
                @JoinColumn(name="groups", referencedColumnName = "id_group")
                @OrderBy("name_of_subject")
                private Set<SubjectsModel> subjects;
                .......
            }

Subjects entity:

                @Entity
                @Table(name = "subjects")
                public class SubjectsModel implements Externalizable {
                    @Id
                    @GeneratedValue(strategy = GenerationType.IDENTITY)
                    private int id;
                    @Column(name = "groups")
                    private Integer groups;
                }

Marks entity:

                @Entity
                @Table(name = "marks")

                public class MarksModel {
                    @Id
                    @GeneratedValue(strategy = GenerationType.AUTO)
                    @Column(name = "id")
                    private int id;
                    @Column(name = "mark")
                    private Integer mark;
                    @Column(name = "id_student")
                    private Integer idStud;
                    @Column(name = "id_subject")
                    private Integer idSubject;
                }

Answer 1

Students entity:

            @Entity
            @Table(name = "students")
            public class StudentsModel implements Externalizable {
                @Id
                @Column(name = "id")
                @GeneratedValue(strategy = GenerationType.IDENTITY)
                private int id;               
                @Column(name = "id_group")
                private Integer idGroup;
                .......
                @OneToMany
                @JoinColumn(name="id", referencedColumnName = "id_student")
                private Set<MarksModel> marks;
                .......
            }

Subjects entity:

                @Entity
                @Table(name = "subjects")
                public class SubjectsModel implements Externalizable {
                    @Id
                    @GeneratedValue(strategy = GenerationType.IDENTITY)
                    private int id;
                    @Column(name = "groups")
                    private Integer groups;
                    @OneToMany
                    @JoinColumn(name="id", referencedColumnName = "id_subject")
                    private Set<MarksModel> marks;


                }

Marks entity:

                @Entity
                @Table(name = "marks")

                public class MarksModel {
                    @Id
                    @GeneratedValue(strategy = GenerationType.AUTO)
                    @Column(name = "id")
                    private int id;
                    @Column(name = "mark")
                    private Integer mark;
                    @ManyToOne(cascade = CascadeType.ALL)
                    @JoinColumn(name = "id_student")
                    private StudentsModel student;
                    @ManyToOne(cascade = CascadeType.ALL)
                    @JoinColumn(name = "id_subject")
                    private SubjectsModel subject;
                }

That's how the mapping should look like, for all your JPA needs of handling those. If order will be relevant use List instead of Set. This however does not give you the JSON structure you dream of out of the box - it can't really due to how the database is constructed. There might (?) be some JSON magic to be done that could make it work, but the only way I see it is through another set of classes for serialization purposes:

public class StudentPOJO implements Serializable {
    private String name;
    private int id;
    private List<SubjectPOJO> subjects = new ArrayList<>();

    public StudentPOJO(StudentsModel model) {
        this.name = model.getName();
        this.id = model.getId();
        this.subjects.addAll(model.getMarks().map(mark -> new SubjectPOJO(mark.getSubject())).collect(Collectors.toList()));
    }
}

and analogically for SubjectPOJO and MarkPOJO.

Answer 2

These could be part of marks class: