Hibernate and Jackson manyToMany relation with extra fields

Question

Recently I've been trying to link a Module table and an Activity table by a Many to Many relationship. This relation welcomes an additional field as below.

-------------------------------------------------------------
| Module        |   ModuleHasActivities |     Activities    |
-------------------------------------------------------------
| id            |  Module_idModule      |   id              |   
| Name          |  Activity_idActivity  |   Name            |
| Observations  |  Day                  |   Price           |
-------------------------------------------------------------

I use JsonManagedReference and JsonBackReference to avoid infinite loops. Unfortunately the result is not the expected one since I don't get the list of modules from the activities and vice versa. The only information returned are the entity fields.

@Entity
@Table(name = "Module")
public class Module implements Serializable {

    @Id
    @GeneratedValue
    @Column(name = "idModule")
    private int id;
    
    // Other fields plus getter and setter

    @OneToMany(mappedBy = "module", cascade = CascadeType.ALL)
    @JsonBackReference
    private Set<ModuleHasActivities> moduleHasActivities = new HashSet<>();
}

@Entity
@Table(name = "Activite")
public class Activite implements Serializable {

    @Id
    @GeneratedValue
    @Column(name = "idActivite")
    private int id;

    // Other fields

    @OneToMany(mappedBy = "activite", cascade = CascadeType.ALL)
    @JsonBackReference
    private Set<ModuleHasActivities> moduleHasActivities = new HashSet<>();
}

@Entity
@Table(name = "Module_has_Activite")
public class ModuleHasActivities implements Serializable {

    @Id
    @ManyToOne
    @JoinColumn(name = "Module_idModule")
    @JsonManagedReference
    private Module module;

    @Id
    @ManyToOne
    @JoinColumn(name = "Activite_idActivite")
    @JsonManagedReference
    private Activite activite;

    @Column(name = "IdJour")
    private int jour;

}

Query modules response:

[
    {
        "id": 1,
        "code": "TT1",
        "title": "Test 1",
        "description": "Nothing",
        "observations": "Nothing",
    }
]

What I expect (something like this) :

[
    {
        "id": 1,
        "code": "TT1",
        "title": "Test 1",
        "description": "Nothing",
        "observations": "Nothing",
        "activities": [
          "day1": 
          {
           //fields of activities
          },
          "day2": 
          {
           //fields of activities
          },
    ]
    }
]

When I debug hibernate's SQL queries, we see that the join is never done. And that's what interests me! Joints to the module table are missing.

A little help would be appreciated,

Thank you

Already visited links : Extra fields for many to many relations

Answer 1

You need to understand the differences between JsonManagedReference and JsonBackReference. See this: https://stackoverflow.com/a/37394318/607637 for more info.

@Entity
public class Module  {
    @JsonBackReference
    private Set<ModuleHasActivities> moduleHasActivities = new HashSet<>();
}

@Entity
public class ModuleHasActivities implements Serializable {
    @JsonManagedReference
    private Module module;
}

Here, if you serialize Module object, you will not get moduleHasActivities field in json but if you serialize ModuleHasActivities object you will get module field.

So, the solution is to flip the @JsonBackReference and @JsonManagedReference as below:

@Entity
public class Module  {
    @JsonManagedReference
    private Set<ModuleHasActivities> moduleHasActivities = new HashSet<>();
}

@Entity
public class ModuleHasActivities implements Serializable {
    @JsonBackReference
    private Module module;
}

Answer 2

Finally, I solved my problem. Instead of using a composite key, I changed my database model to use a primary id key and 2 references to my concerned tables. Mapping is easier and I get my information.