Need help understanding foreign keys - spring

Question

I have a message class with a foreign key that should be the id of the author who wrote the message. As I understand it, the foreign key in the message class should look like the class below.
Setting the foreign key as the author object as below, seems absurd because instead of a short and human readable id like "5", I get a very long string in the database that isn't human readable.
I'm missing something, right?
message class:
public class Message {
...
private Author author; // this is the foreign key
...
#ManyToOne
#JoinColumn(name = "USERNAME")
public User getAuthor() {
return author;
}
public void setAuthor(Author author) {
this.author = author;
}
...
Creating the message object to be saved:
Author author = ...
message.setAuthor(author);

Answer 1

Assuming you are simply looking for a Many-to-One unidirectional relationship
#Entity
public class Message {
...
#ManyToOne
#JoinColumn(name="USERNAME")
private Author author;
#Entity
public class Author {
#Id
#GeneratedValue
#Column(name="USERNAME")
private Long USERNAME;

Answer 2

You do not post the annotations from the Author class. And it is highly probable that You are missing annotations on the Author class site. Nevertheless look below:
#Entity
#Table(name = "bill")
public class BillModel {
#Id
#GeneratedValue(strategy = GenerationType.SEQUENCE)
#Column(name = "bill_id")
private Integer billId;
#ManyToOne(fetch = FetchType.EAGER)
#JoinColumn(name = "fk_shop_id")
private Shop shop;
// getters and setters
}
and class Shop
#Entity
#Table(name = "shop")
public class Shop {
#Id
#GeneratedValue
#Column(name = "shop_id")
private Integer shopId;
#Column(name = "shop_name")
private String shopName;
#OneToMany(mappedBy = "shop", fetch = FetchType.EAGER, cascade = CascadeType.ALL)
private Set<BillModel> billModels = new HashSet<BillModel>();
// getters and setters
}