Calling the function morphTo returns null - php

Question

right now i am stuck in this not really knowing what to do (maybe is really easy, but i don't see how to do it).
I have this model of users:
<?php
namespace App;
use Illuminate\Contracts\Auth\MustVerifyEmail;
use Illuminate\Foundation\Auth\User as Authenticatable;
use Illuminate\Notifications\Notifiable;
class User extends Authenticatable
{
use Notifiable;
protected $table = 'tcn_user';
// Time for the relations
public function user(){
return $this->morphTo(__FUNCTION__, 'type_user', 'id_user');
}
/**
* The attributes that are mass assignable.
*
* #var array
*/
protected $fillable = [
'username','permision', 'first_name', 'surname', 'us_password', 'email'
];
/**
* The attributes that should be hidden for arrays.
*
* #var array
*/
protected $hidden = [
'us_password', 'remember_token',
];
/**
* The attributes that should be cast to native types.
*
* #var array
*/
protected $casts = [
'email_verified_at' => 'datetime',
];
}
Being the problem (I suppose) in the function user(). I call it but it returns null when doing it, the variables for the childs are there, so I really don't know what is wrong. But for doubts I have here the code for one of them so you can see it.
<?php
namespace App;
use Illuminate\Database\Eloquent\Model;
class Teacher extends Model
{
protected $table = 'teacher';
//In first place let's do the relations of the model
public function user(){
return $this->morphOne(User::class, 'user', 'id_user');
}
public function schedule(){
return $this->hasMany(ServiceSchedule::class, 'id_teacher');
}
public function session(){
return $this->hasMany(DaySession::class, 'id_teacher');
}
}
Being the important one the function user().
If you need for information please let me know, is my first time asking here, so i really don't know if I am doing it well.
PD: Okey, sorry for the things i have forgotten to add.
This being to code about how i am calling the Teacher:
public function detail($id){
$user = User::find($id);
if(is_object($user)){
$data = array(
'code' => 200,
'status' => 'success',
'user' => $user,
'subuser' => $user->user
);
}else{
$data = array(
'code' => 404,
'status' => 'error',
'message' => 'El usuario no existe'
);
}
return response()->json($data);
}
And the relation it have, well, we have User, the parent "class" with the information all the users need to have, and after that we have the "childs" like Teacher, Student, Parent, all of then with different information.
And responding to Rwd, no, i don't have a camp in the database with the name user, is more, the table user is named tcn_user (Because user is a keyword in mysql)