Hibernate Triple Multi-to-Many Join

Question

I need some help modeling a table that has a many to many relationship to another table which has a many to many relationship to a third table.

To illustrate, say I have three tables relating to automobiles: Dealership, Make, and Model. A Dealership has a many to many relationship with Make (ie: Honda, Ford, etc) and a Make has a many to many relationship with the Model (ie: Passport, Accord, etc). (Note, in real life I'm sure Make's and Model's don't have a many to many relationship, but I'm using this relationship for illustration purposes). Given a particular Dealership, I'd like to get all the Make's they carry, and from each Make I'd like to get all the Model's of that Make that the Dealership carries. They may not carry every Model of a particular Make, so it should only return Model's they carry.

Dealership
----------------
1 - Motor Experts
2 - Auto Sales

Make
----------------
1 - Honda
2 - Ford

Model
----------------
1 - Passport
2 - Accord
3 - Taurus

I know how to do a many-to-many relationship between Dealership and Make and a many-to-many relationship between a Make and Model. The problem is I don't know how to relate only the Models that the Dealership carries.

What is the best way to model the join tables and what would the JPA/Hibernate annotations be on my java model objects?

Answer 1

I'd almost say that you're missing an entity called "car" which would have "Make, Model and Year". Dealerships would then have Cars. You would no longer have the 3-way many-to-many. But given that it is just an example you could model the relationship as its own entity, let's say "Inventory", which would have a Dealership, Make, Model and any additional attributes associated with it that you'd like (eg Year, Quantity)

Inventory (Dealership, Make, Model)
---------
1, 1, 1  --  (Motor Exports, Honda, Passport)
1, 2, 3  --  (Motor Exports, Fort, Taurus)
etc...

In Hibernate you model Inventory as its own entity with an embedded primary key. In abbreviated form, it would look like this:

@Entity
public class Inventory {
    @EmbeddedId 
    public InventoryId id;
    // additional attributes

    @MapsId("dealership")
    @ManyToOne
    public Dealership dealership;

    @MapsId("make")
    @ManyToOne
    public Make make;

    @MapsId("model")
    @ManyToOne
    public Model model;
}

@Embeddable
public class InventoryId implements Serializable {
    @ManyToOne public Dealership dealership;
    @ManyToOne public Make make;
    @ManyToOne public Model model;

    // Don't forget to implement equals and hashcode
}

@Entity
public class Dealership {
    @Id @GeneratedValue public Long id;
    @Basic public String name;

    @OneToMany(mappedBy="dealership") public List<Inventory> inventory;
}

// classes Make and Model follow the same pattern as Dealership

Then to run a query for all models that a particular dealership has in inventory you would use:

List<Model> stock = em.createQuery(
        "select distinct inv.model "+
        "from Dealership d "+
        "join d.inventory inv "+
        "where d.name = :dealer",
        Model.class)
    .setParameter("dealer", "Motor Exports")
    .getResultList();

Answer 2

The dealership-model relationship is not direct; it only exists by virtue of the dealership-make and make-model relationships. This, it's not appropriate to specify it as a property of the dealership. Rather, you should do a JPQL query to find models available from a dealership.