Hibernate load another column related to a foreign key

I am having these two relations:

ACTION(ID, AMOUNT, BY) 
USER(ID, NAME)

//BY is a foreign key that references USER(ID)

In my JAVA project I am having something like this:

@Entity
@Table("action")
class Action{
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  @Column(name = "id")
  private int id;

  @Column(name = "amount")
  double amount;

  @Column(name="by")
  int userId;

  @???
  String userName;
}

and

@Entity
@Table("user")
class User{
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  @Column(name = "id")
  private int id;

  @Column(name = "name")
  String name;
}

What I want to know is how to link my Action class so that it loads the correct userName which is referred by by (userId) column?

Thanks in advance!

You can add User in Action class and @JoinColumn with by

class Action{
  ...
  @ManyToOne(fetch = FetchType.LAZY)
  @JoinColumn(name = "by")
  User user;
}

class User{
 ...
  @OneToMany(mappedBy = "user", cascade = CascadeType.ALL)
  private List<Action> actions;
}

You're already keeping the userId information in your Action class - so either remove the userName field, and when you need that information fetch it separately based on the userId. So your Repository or DAO class can expose a method named: findUsernameBy(int userId) returning a String.

Another way is to implement OneToOne relation with a user entity:

@Entity
@Table("action")
class Action{
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  @Column(name = "id")
  private int id;

  @Column(name = "amount")
  double amount;

  @OneToOne
  @JoinCoumn(name="by")
  private User user;
}

If you have many actions per user, replace:

@Column(name="by")
int userId;

@???
String userName;

with

@ManyToOne
@JoinColumn(name = "by")
private User user;

You can also add:

// Cascade to propagate wanted actions to many side
@OneToMany(mappedBy = "user", cascade = CascadeType.ALL)
private List<Action> actions;

to your User entity to have birectional mapping. So to list all Users actions. In addition to adding to cascade and adding action to users actions you need to set action's user or saving will not work correctly. In other words you need to set both ends of mapping correctly.