Spring boot JPA many to many with extra column insert and update issue

Question

Here is my initial question.

Spring Data JPA Many to Many with extra column User and Roles

Now I have the right tables created, but can't make it work for the update.

Here is the code:

User.java

@Entity
@Table(name = "users")
public class User {

  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Long id;

  private String username;

  @OneToMany(mappedBy="user", cascade = CascadeType.ALL, orphanRemoval = true,  fetch = FetchType.LAZY)
  private List<UserRole> roles;
  // getters and setters
}

Role.java

@Entity
@Table(name = "roles")
public class Role {

  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Long id;

  private String name;
  // getters and setters
}

UserRole.java

@Entity
@Table(name = "users_to_role")
public class UserRole  implements Serializable {

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

  @Id
  @ManyToOne
  @JoinColumn(name = "role_id")
  private Role role;

  private Date createdAt;

  public UserRole(){}

  public UserRole(User user, Role role, Date date) {
    this.user = user;
    this.role = role;
    this.createdAt = date;
  }

  // getters and setters
}

Controller

@RestController
public class APIController {

  @Autowired
  RoleRepository roleRepository;

  @Autowired
  UserRepository userRepository;


  @ResponseBody
  @RequestMapping(value = "create", method = RequestMethod.GET)
  public String create(){

    //Insert test - WORKING BUT NOT SURE IF ITS RIGHT WAY
    List<UserRole> userRoles = new ArrayList<>();
    Role role = roleRepository.getOne((long) 1);

    //Create user
    User user = new User();
    user.setUsername("test");

    //Create userRole
    userRoles.add(new UserRole(user, role, new Date()));
    user.setRoles(userRoles);

    userRepository.save(user);

    return "created";

  }



  @ResponseBody
  @RequestMapping(value = "edit", method = RequestMethod.GET)
  public String edit(){

    //Edit test - NOT working
    List<UserRole> userRoles = new ArrayList<>();

    Role role = roleRepository.getOne((long) 2);

    //get user from db
    User user = userRepository.getOne((long) 1);

    //Create userRole
    userRoles.add(new UserRole(user, role, new Date()));

    // WAS FIRST ATTEMPT using user.setRoles(userRoles); but got error and use
    //https://stackoverflow.com/questions/9430640/a-collection-with-cascade-all-delete-orphan-was-no-longer-referenced-by-the-ow
    //user.setRoles(userRoles);

    user.getRoles().clear();
    user.getRoles().addAll(userRoles);


    userRepository.save(user);

    return "done";
  }
}

I am getting this error:

com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Column 'user_id' cannot be null