Correct way to do a join table with an additional attribute in JPA?

Question

I've got the following entities (simplified for the purpose of this thread):

Product.java

@Entity
@Table(name = "Product")
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @Column(nullable = false)
    private String name;
    
    // getters, setters,...
}

Ingredient.java

@Entity
@Table(name = "Ingredient")
public class Ingredient {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @Column(nullable = false)
    private String name;

    // getters, setters,...
}

And I'd like to add a many to one relationship to represent a product being made of certain amount of some ingredients.

Something like the following would only hold the information about which ingredients a product is made of, but it wouldn't tell me about the amount each ingredient it contains. What's the most JPA way to add a simple amount attribute to the join table?

Product.java

@Entity
@Table(name = "Product")
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @Column(nullable = false)
    private String name;

    @ManyToMany
    @JoinTable(
        name = "Product_Ingredient",
        joinColumns = @JoinColumn(name = "product_id"),
        inverseJoinColumns = @JoinColumn(name = "ingredient_id")
    )
    private List<Ingredient> ingredients;

    // getters, setters,...
}

Answer 1

It can be done with an additional entity containing the amount and two many-to-one relationships. One to Product other to Ingredient. The table will consist of two ids and the amount.

Solution

Additional entity:

@Entity
class ProductIngredient {

    @Id
    Long id;

    @ManyToOne
    @JoinColumn(name = "product_id")
    Product product;

    @ManyToOne
    @JoinColumn(name = "ingredient_id")
    Ingredient ingredient;

    BigDecimal amount;
    
    // ...
}

The relationships in existing entities:

class Product {

    // ...

    @OneToMany(mappedBy = "product")
    Set<ProductIngredient> productIngredients;

    // ...
}
class Ingredient {

    // ...

    @OneToMany(mappedBy = "ingredient")
    Set<ProductIngredient> productIngredients;

    // ...
}