cssd-back/app/Message.php

<?php

namespace App;
use Illuminate\Database\Eloquent\Model;

class Message extends Model
{
  public $fillable = ['title', 'text', 'recipient_user_id', 'sender_user_id'];
  public function sender_user()
  {
      return $this->belongsTo('App\User', 'sender_user_id');
  }
  public function recipient_user()
  {
      return $this->belongsTo('App\User', 'recipient_user_id');
  }
}