How to get all children of a parent if parent_id field is null

i have section table have this fields (id ,parent_id ,lft,rght,name)
when sent $id it will check by this id the parent_id field,then if it
is null,it will search for his children,how to do this..

i write this code but it is wrong

function view($id = null) {

// find section in database
$section = $this->Section->findById($id);
// set the section for the view
$this->set(compact('section'));
// if find id check parent_id field and if its is null then find his
children
if ($section['parent_id']== null){
$this->Section->Article->children($id) ;
}else{
// else find parent only
$this->Section->findbyid(parent);

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