app\models\Contact::user PHP Method

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

 /**
  * Associate Contact with User.
  *
  * @param Contact $contact
  * @param User    $user
  *
  * @return App\Models\Contact
  */
 public function linkToUser(Contact $contact, User $user)
 {
     $contact->user()->associate($user->id);
     $contact->save();
     return $contact->fresh();
 }