问如何使用一对多的关系在Laravel中保存所选列表？

EN

<div class = "form-group {{ $errors->has('role') ? ' has-error' : '' }}">

<label for = "role" class = "control-label">Role</label>

<select class = "form-control" name = "role">

    <option selected disabled>Please select role</option>
    <option value = "role1">Supplies Officer</option>
    <option value = "role2">Admin Officer</option>
    <option value = "role3">Attorney</option>
    <option value = "role4">Chairman</option>

</select>

 @if ($errors->has('role'))
    <span class = "help-block">{{ $errors->first('role') }}</span>
 @endif

INSERT INTO `users`(`role_id`, `email`, `username`) VALUES (1,'francis@gmail.com','francis')

class User extends Model implements AuthenticatableContract
{
use Authenticatable;

protected $table = 'users';

protected $fillable = [
    'username',
    'email',
    'password',
    'role_id',
];

public function role()
{
    return $this->belongsTo('Role');
}
}

class Role extends Model
{
protected $table = 'roles';

public function user()
{
    return $this->hasMany('User');
}
}

public function postRegister(Request $request)
{
    $this->validate($request, [
                            //This will be unique in users table
        'email' => 'required|unique:users|email|max:255',
        'username' => 'required|unique:users|alpha|max:20',
        'password' => 'required|min:6',
        'role' => 'required',
    ]);

    $email = $request['email'];
    $username = $request['username'];
    $password = bcrypt($request['password']);
    $role = $request['role'];

    $user = new User();
    $user->email = $email; //Accessing properties of Model
    $user->username = $username;
    $user->password = $password;


    $user->save();

}