Spring Data Jpa:如何让另一个表的外键是唯一的？

@Entity
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String username;

    // getters and setters
}

@Entity
public class UserRole {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne
    @JoinColumn(name = "userId", unique = true)
    private User user;

    private String role;

    // getters and setters
}

public interface UserRepository extends JpaRepository<User, Long> {
}

public interface UserRoleRepository extends JpaRepository<UserRole, Long> {
}

CREATE TABLE user (
    id BIGINT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(255) NOT NULL
);

CREATE TABLE user_role (
    id BIGINT AUTO_INCREMENT PRIMARY KEY,
    user_id BIGINT UNIQUE,
    role VARCHAR(255) NOT NULL,
    FOREIGN KEY (user_id) REFERENCES user(id)
);

@Transactional
public void addUserRole(Long userId, String role) {
    if (userRepository.existsById(userId)) {
        UserRole userRole = new UserRole();
        userRole.setUser(userRepository.findById(userId).orElseThrow());
        userRole.setRole(role);
        userRoleRepository.save(userRole);
    } else {
        throw new IllegalArgumentException("User not found");
    }
}