Question

I have a Class Site that represents a website and a Class User. A Site can have multiple Users.

class Site {

    private int site_ID;

    @OneToMany // with a join table
    private List<User> users;
    // ...
}

class User {

    private int user_ID;

    private String name;

    private String lastname;

    private String username;

    private String password;

}

I want to allow same username to exist on all Sites, but only one by site.

Site/User/username
1   /1   /username1
1   /2   /username2
2   /3   /username1

How can I do that?

Answer 1

Let the user have a Site reference:

@ManyToOne(optional=false)
private Site site;

Now add the constraint to user:

@Table(uniqueConstraints = {
    @UniqueConstraint(columnNames = { "username", "site" })})
@Entity
public class User{
// etc
}

You will also have to change the Site mapping:

@OneToMany(mappedBy="site")
private List<User> users;