JPA如何在@OneToMany关系的列上添加唯一约束,例如在用户名上

@Entity
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @Column(unique = true) // 添加唯一约束
    private String username;
    
    // other fields and relationships
}

@Entity
public class Role {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    // other fields and relationships
}

@Entity
public class UserRole {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @ManyToOne
    @JoinColumn(name = "user_id")
    private User user;
    
    @ManyToOne
    @JoinColumn(name = "role_id")
    private Role role;
    
    // other fields and relationships
}