Avoid duplication on manyToMany relationships with 3 tables

I’m trying to implement a User system with roles and privileges. So I have 5 tables:

• users
• roles
• privileges
• users_roles
• roles_privileges

My class user owns the relationship users_roles and the class roles owns the relationship roles_privileges.

To describe my problem lets say I have the following in my database (simplified):

• users: user
• roles: 0, 1
• privileges: a, b, c, d
• users_roles: (user, 0), (user, 1)
• roles_privileges: (0,a), (0,b), (0,c), (1,d)

Now if I retrieve the roles from the user I get the following (simplified): [0, 0, 0, 1]

On the list the role 0 appeared 3 times and the role 1 appeared 1. Which happends to correspond to the number of privileges each of the roles have.

I don’t understand why this is happening. Also this only happends when I retrive the user with the function findById from the user repository. When I retrieve the roles with the “authenticated user” this doesn’t happend.

Here’s the code of the 3 entities (simplified):

USER

@Entity @Table(name = "user")
public class User implements UserDetails {
    
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @ManyToMany(fetch = FetchType.EAGER) @JoinTable( 
        name = "users_roles", 
        joinColumns = @JoinColumn(name = "user_id", referencedColumnName = "id"), 
        inverseJoinColumns = @JoinColumn(name = "role_id", referencedColumnName = "id")
    ) 
    private Collection<Role> roles;
    
    @JsonIgnore public Collection<Role> getRoles() { return this.roles; } // This is the function that is returning duplicateds
}

ROLE

@Entity  @Table(name = "role")
public class Role {
    
    @JsonView(Views.Basic.class)
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @ManyToMany(mappedBy = "roles")
    private Collection<User> users;
    
    @ManyToMany(fetch = FetchType.EAGER) @JoinTable(
        name="roles_privileges",
        joinColumns = @JoinColumn(name = "role_id", referencedColumnName = "id"),
        inverseJoinColumns = @JoinColumn(name = "privilege_id", referencedColumnName = "id")
    )
    private Collection<Privilege> privileges;
    
}

PRIVILEGE

@Entity  @Table(name = "privilege")
public class Privilege {
    
    @JsonView(Views.Basic.class)
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @ManyToMany(mappedBy = "privileges")
    private Collection<Role> roles;
}

And here is the code of the endpoint that is trying to retrieve the roles from a user:

@RequestMapping(value = "/users/{id}", method = RequestMethod.GET)
@ResponseBody @JsonView(Views.Basic.class)
public Response getRolesFromUser(@PathVariable Long id, @AuthenticationPrincipal final User authUser, HttpServletRequest request) {
    if(!authUser.hasRole("admin")) throw (new ResponseStatusException(HttpStatus.UNAUTHORIZED, "You don't have admin privilegies."));
    
    User user = userService.findById(id);
    if(user == null) throw(new ResponseStatusException(HttpStatus.NOT_FOUND, "User not found."));
    
    return Response.builder().status(HttpStatus.OK).message("User's roles retrieved.").data(user.getRoles()).request(request).build();
}

user.getRoles() returns what I told earlier but authUser.getRoles() works fine. I don’t really need to resolve this problem since this is only intended to be used by the administrator to see other users roles and modify them (but I could always do that on the database). But still is a strange behaviour that I cannot understand and I would like to be able to administrate roles from the application and not from the database.

Answer

As a work arround, I tried to use Set instead of Collection for the user roles. It solved my problem.

This work arround, doesn’t solve the question “Why is behaving this way when using Collection?”.