/**
  * @param mixed $tenant
  * @param mixed $user
  * @return mixed
  */
 public function create($tenant, $user)
 {
     $model = TenantUser::find()->where(['tenant_id' => $tenant->id, 'user_id' => $user->id])->one();
     if (!$model) {
         $model = new TenantUser();
         $model->tenant_id = $tenant->id;
         $model->user_id = $user->id;
         if ($model->save()) {
             return $model;
         }
         return null;
     }
     return $model;
 }