org.hibernate.MappingException: Could not determine type for custom object type

Spring boot 2.3

Entity Cart has many entites CartItem.

So here my models:

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


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


@Entity
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int id;
    @NotNull
    private String name;
    @ElementCollection
    private Set<String> images;

But when I 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 javax.persistence.PersistenceException: [PersistenceUnit: default] Unable to build Hibernate SessionFactory; nested exception is org.hibernate.MappingException: Could not determine type for: com.myproject.eshop_orders.api.model.Product, at table: cart_item, for columns: [org.hibernate.mapping.Column(product)]

You are using Product in CartItem without defining any relation like @OneToOne. So JPA treats product as column in CartItem table and can not determine compatible type for product for database column. That's why error states that

Could not determine type for: com.myproject.eshop_orders.api.model.Product, 
at table: cart_item, for columns: [org.hibernate.mapping.Column(product)

May be you have @OneToOne relation with Product in CartItem table.

@OneToOne
private Product product;