How to model a three-way relationship in a JPA Spring Boot micro service with a MySQL back end

Question

I have created a Spring Boot JPA micro service with a MySQL back end. I have two simple entities in my model, users and skills. These are used to represent the skills base within my organisation. Each user may have many skills and so I have used a one to many join to model this and can successfully assign skills to users. In my database, I can see that a users_skills table has been created by JPA to achieve this.

However, I am now struggling because, for each skill that a user has, an experience level needs to be specified (eg basic, advanced, expert) and I am unsure how to achieve this. I'm not sure whether 'levels' should just be an enum type within the Skill entity, or perhaps it should be a separate entity in its own right? Could I configure JPA so that it generates a users_skills_levels table which would represent this three-way relationship? Any advice would be most welcome!

These are my Entity classes: -

@Entity
@Table(name = "users")
public class User {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Integer id;

    private String name;

    private String email;

    @OneToMany(
            cascade = CascadeType.ALL,
            orphanRemoval = true
    )
    private Set<Skill> skills = new HashSet<>();

    getters and setters
}

@Entity
@Table(name = "skills")
public class Skill {

    @Id
    @GeneratedValue(strategy= GenerationType.IDENTITY)
    private Integer id;

    private String name;

    getters and setters
}

Answer 1

That's not possible what you try to achieve.

You should create an Entity for the users_skills_levels. Eg UserSkillLevel This entity will then have a ManyToOne relationship to User and a ManyToOne relationship to Skills plus the attribute level.

The User has a collection of UserSkillLevel and the Skill entity as well.

Please find a more in-depth example here:

https://thoughts-on-java.org/many-relationships-additional-properties/