Jpa ManyToMany Self Reference without auxiliar Join Table

Question

I'd like to tell entity to join with itself for a field that it's not unique in both directions, but it looks like all examples in internet use a Join table or either are pretty old.

Person Denormalized Table:

PersonId (Pk) | RoleId | ParentRoleId
  1               1         NULL
  2               1         NULL
  3               2          1
  4               2          1

Person Entity (With mappings that seem to load an empty list):

@Column
private Long personId;

@Column
private Long roleId;

@Column
private Long parentRoleId;

@ManyToMany
@JoinColumn(name = "parentRoleId", referencedColumnName = "roleId", updatable = false, insertable = false)
private List<Person> personsWithParentRole;

@ManyToMany
@JoinColumn(name = "roleId", referencedColumnName = "parentRoleId", updatable = false, insertable = false)
private List<Person> personsWhoseRoleHasCurrentPersonRoleAsParent;

I would like to know if there's a way to map my case. I know it's not the best arch or the most performant, and that one can use a different approach, but please I am just wondering about that particular solution. This is a simplification from a most complex case.

Answer 1

I think is definitly a bad idea to avoid a join table in your case. Your current solution is the best.

I think you need something like this:

public class Person {
    @Id
    private long id;

    @JoinTable(name = "person_links", joinColumns = {
            @JoinColumn(name = "subordinate", referencedColumnName = "id", nullable = false)}, inverseJoinColumns = {
            @JoinColumn(name = "manager", referencedColumnName = "id", nullable = false)})
    @ManyToMany
    private List<Person>subordinates;


    @ManyToMany(mappedBy = "subordinates")
    private List<Person> managers;

}

Answer 2

Disclaimer : this answer is not definitive. I write the answer for readibility sake and it is meant to be improved along the comment with OP. Furthermore, code is not tested.

---

Database design

for the answer, I'll avoid the join table and assume that table is designed as followed:

• there are two tables: person and role with respective primary key columns PersonId and RoleId
• in the person table, RoleId and ParentRoleId are foreign keys referring to the same role.RoleId
• other columns in role table (eg relationship between roles) are irrelevant for the question

JPA

Entities

Entities follow table structure. The Role entity would be a basic entity:

@Entity
public class Role{

    // ---- JPA attributes
    @Id
    // ...
    @Column(...)
    private Long roleId;

    @OneToMany(mappedBy = "role")
    private List<Person> personsWithThisRoleAsPrimaryRole;

    @OneToMany(mappedBy = "parentRole")
    private List<Person> personsWithThisRoleAsParentRole;

    // ---- Constructor
    public Role(){
        // your initialisation

        // initialise list to avoid NullPointerException
        this.personsWithThisRoleAsPrimaryRole = new ArrayList<>();
        this.personsWithThisRoleAsParentRole = new ArrayList<>();

    }

    // getters & setters
}

The trick for bypassing the join table would be leveraging the @OneToMany relationship with a transient attribute:

@Entity
public class Person{

    // ---- JPA attributes
    @Id
    // ...
    @Column(...)
    private Long personId;

    @ManyToOne
    @JoinColumn(name = "RoleId")
    private Role role;

    @ManyToOne
    @JoinColumn(name = "ParentRoleId")
    private Role parentRole;

    // ---- Transient attributes
    @Transient
    private List<Person> personsWithParentRole;

    @Transient
    private List<Person> personsWhoseRoleHasCurrentPersonRoleAsParent;

    // ---- Constructor

    public Person(){
        // your initialisation

        // initialise list to avoid NullPointerException
        this.personsWithParentRole = new ArrayList<>();
        this.personsWhoseRoleHasCurrentPersonRoleAsParent = new ArrayList<>();
    }

    @PostLoad
    public void postLoad(){
        // during JPA initialisation, role and parentRole have been defined
        // if the value exist in the database. Consequently, we can fetch some
        // interesting info:
        if(role != null){
            personsWithParentRole.addAll(role.getPersonsWithThisRoleAsParentRole());
        }
        if(parentRole != null){
            personsWhoseRoleHasCurrentPersonRoleAsParent.addAll(parentRole.getPersonsWithThisRoleAsPrimaryRole());
        }
    }

    // getters and setters for JPA attributes

    // getters for transient attributes. It doesn't make sense to create the setters for the transient list here.
}

Transient attributes

Transient attributes have to be used with care as I encountered many fancy problems. However, they are useful as you may fetch the persons list once. If you had something like:

public List<Person> getPersonsWithParentRole{
    if(role != null){
        return role.getPersonsWithThisRoleAsParentRole();
    }
}

public List<Person> getPersonsWithParentRole{
    if(parentRole != null){
        return parentRole.getPersonsWithThisRoleAsPrimaryRole();
    }
}

It should also work but performance wise, it may engender additional irrelevant calculations.

For your example

To see if it should work, let's do a paper+pen like draft:

Person table

Person | Role | ParentRoleId
------ | ---- | ------------
   1   |   1  |     null
   2   |   1  |     null
   3   |   2  |      1
   4   |   2  |      1

Role table

Role | Additional Columns
---- | ----------------
  1  |      ...
  2  |      ...

Entity-wise

Person entity without considering the @PostLoad and transient lists:

Person | Role | ParentRoleId
------ | ---- | ------------
   1   |   1  |     null
   2   |   1  |     null
   3   |   2  |      1
   4   |   2  |      1

the role entities with the @OneToMany relationships:

Role | PersonsWithThisRoleAsPrimaryRole | PersonsWithThisRoleAsParentRole
---- | -------------------------------- | -------------------------------
  1  |           [1, 2]                 |            [3, 4]
  2  |           [3, 4]                 |            [empty]

Consequently, after @postLoad , you'll have:

Person | Role | ParentRoleId | PersonsWithParentRole | PersonsWhoseRoleHasCurrentPersonRoleAsParent
------ | ---- | ------------ | --------------------- | --------------------------------------------
   1   |   1  |     null     |       [3,4]           |                 [empty]
   2   |   1  |     null     |       [3,4]           |                 [empty]
   3   |   2  |      1       |       [empty]         |                 [1, 2]
   4   |   2  |      1       |       [empty]         |                 [1, 2]

/!\\ Be careful about initialisation stuff (Lazy initialisation can be tricky) /!\\

Hope this helps