ManyToManyToMany - Join three tables with Hibernate annotations

Question

At first I thought this solution could solve my problem:

@Entity
public class User {

    @JoinTable(name = "user_permission",
        joinColumns = @JoinColumn(name = "user_id"),
        inverseJoinColumns = @JoinColumn(name = "permission_id"))
    @MapKeyJoinColumn(name = "project_id")
    @ElementCollection
    private Map<Project, Permission> permissions = new HashMap<>();

}

@Entity
public class Project {
    ...
}

@Entity
public class Permission {
    ...
}

But in this implementation, only one can be Permissionset to Project. I would like to fulfill the possibility of setting multiple permissions for a project so that the following can be true:

| user_id | project_id | permission_id |
|---------|------------|---------------|
| 1       | 1          | 1             |
|---------|------------|---------------|
| 1       | 1          | 2             |
|---------|------------|---------------|
| 1       | 2          | 1             |
|---------|------------|---------------|
| 1       | 2          | 2             |
|---------|------------|---------------|
| 2       | 1          | 1             |
|---------|------------|---------------|
| 2       | 1          | 2             |
|---------|------------|---------------|
| 2       | 2          | 1             |
|---------|------------|---------------|
| 2       | 2          | 2             |

Answer 1

You can use the object allocated for your relationship table. This is how we declare relationships with their own attributes, for example.
This will result in the following implementation:

@Entity
@IdClass(PermissionAssignation.class)
public class PermissionAssignation {

     @Id
     @ManyToOne
     @JoinColumn(name="user_id")
     private User user;

     @Id
     @ManyToOne
     @JoinColumn(name="project_id")
     private Project project;

     @Id
     @ManyToOne
     @JoinColumn(name="permission_id")
     private Permission permission;
     ...
}

, : Hibernate no PK

, PK ( ). , EmbeddedId.

, , Set<PermissionAssignation> ( List, /):

 @Entity
 public class User {

      @OneToMany(mappedBy="user")
      private Set<PermissionAssignation> permissions;

 }