Can't create entity object with map with custom object

Question

Spring boot 2.5

I has entity Cart. One cart has many entities Product. In one cart has many products. So I try with @JoinColumn but I get error:

@Entity
public class Cart {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int id;
    @OneToMany(mappedBy = "cart", fetch = FetchType.EAGER,
            cascade = CascadeType.ALL)
    private Map<Product, Integer> products;
}


import javax.persistence.*;

@Entity
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int id;
    @ManyToOne(fetch = FetchType.EAGER, optional = false)
    @JoinColumn(name = "cart_id", nullable = false)
    private Cart cart;
}

But when I try to run application I get error:

Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'mvcConversionService' defined in class path resource [org/springframework/boot/autoconfigure/web/servlet/WebMvcAutoConfiguration$EnableWebMvcConfiguration.class]: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.format.support.FormattingConversionService]: Factory method 'mvcConversionService' threw exception; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'cartRepository' defined in com.myproject.eshop_orders.repo.CartRepository defined in @EnableJpaRepositories declared on JpaRepositoriesRegistrar.EnableJpaRepositoriesConfiguration: Cannot resolve reference to bean 'jpaMappingContext' while setting bean property 'mappingContext'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'jpaMappingContext': Invocation of init method failed; nested exception is org.hibernate.AnnotationException: Use of @OneToMany or @ManyToMany targeting an unmapped class: com.myproject.eshop_orders.api.model.Cart.products[java.lang.Integer]

Answer 1

If you want a map you need to use @MapKeyJoinColumn. Something like this

@Entity
public class Cart {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int id;

    @MapKeyJoinColumn(name="PRODUCT_ID")
    @OneToMany(mappedBy = "cart", fetch = FetchType.EAGER,
            cascade = CascadeType.ALL)
    private Map<Product, Integer> products;
}

Answer 2

One cart has many different products. Also I need to store quantity of products in cart (value in map)

Here my solution:

@Entity
public class Cart {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int id;
     @ElementCollection
    @CollectionTable(name = "cart_products", joinColumns = @JoinColumn(name = "cart_id"))
    @MapKeyJoinColumn(name = "product_id")
    @Column(name = "product_qantity")
    private Map<Product, Integer> products;

}


import javax.persistence.*;

@Entity
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int id;
    @ManyToOne(fetch = FetchType.EAGER, optional = false)
    @JoinColumn(name = "cart_id", nullable = false)
    private Cart cart;
}

Is this a correct solution?