laravel - update parent timestamps when updating child model in polymorphic relationship -

in laravel 5.1 have 2 models.a city model , photo model.
there polymorphic relation between city , photo. while updating city's photo with

$city->photos()->updateorcreate($attributes,$values) 

the child time stamps updates. parent model's timestamp , city in case, not update accordingly , should manually call

$city->touch() 

how can update parent model's timestamp when touching child model in laravel?

for polymorphic relations

class photo extends eloquent {    protected $touches = ['city'];    public function city() {     return $this->morphto() // add function if not done   } }  class city extends eloquent {    public function photos() {     return $this->morphmany(app\photo::class, 'city');   } } 

in case, when photo updated, touches parent (city in situation).

hope helps.