laravel relationship one to many

before anything ,Highly appreciated in advance for anyone to help me. i have to model user:

public function posts(){
return $this->hasMany('App\Post');}

post:

public function user(){
return $this->belongsTo('App\user');}

my post have column "user_id" so every post have owner "user_id".

here is my question: i want to delete user but nothing happen to related post. right now i can do this ,but my problem is post_id column have id belongs to deleted user. i want to change that to "null" or "0".

via FarbodKain