How to add column with instant in many-to-many mapping?

Question

everyone! I have to tables "market" and "customer" with many-to-many mapping. Hibernate automatically create table post_comment table with id and then I can get objects and everything ok...but I need the date when a customer by something in the market

market class

@Entity
@Setter
@Getter
@NoArgsConstructor
public class Market{

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @ElementCollection(targetClass = Customer.class, fetch = FetchType.EAGER)
    @CollectionTable(name = "market_customer", joinColumns = @JoinColumn(name = "market_id"))
    private Set<Customer> banks = new HashSet<>();
    
    ............
}

market class

@Entity
@Setter
@Getter
@NoArgsConstructor
public class Customer{

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;
    
    ............
}

I can create automatically marcet_customer with only ids but I can't understand how to add date and most importantly how to get it by JPA repository...

Answer 1

That's the thing with the Hibernate @ManyToMany - it's not meant to have extra columns nor to be queryable (very well). Maybe you want to go with a separate Entity with a composite primary key:

@Entity(name = "MarketCustomer")
@Table(name = "market_customer")
public class MarketCustomer {

    @EmbeddedId
    private MarketCustomerId id;

    @ManyToOne(fetch = FetchType.LAZY)
    @MapsId("marketId")
    private Market market;

    @ManyToOne(fetch = FetchType.LAZY)
    @MapsId("customerId")
    private Customer customer;

    ...
}

@Embeddable
public class MarketCustomerId implements Serializable {

    @Column(name = "market_id")
    private Long marketId;

    @Column(name = "customer_id")
    private Long customerId;

    ...
}

A good article on this https://vladmihalcea.com/the-best-way-to-map-a-many-to-many-association-with-extra-columns-when-using-jpa-and-hibernate/ . With https://bootify.io you can create your schema online as well (no composite primary key, but a separate primary column is doing fine as well).