get full category structure by post id

Question

I have three level category structure as bellow.

- parent 1
  - parent sub 1
    - child 1
    - child 2
  - parent sub 2
    - child 3
    - child 4

Unfortunately, some of posts doesn't fill correctly which means it ticks only the sub parent level or child level.

I have a specific requirement to get whole category structure by post id. So I used following code to retrieve the ids,

wp_get_post_categories( $post_id )

This is only giving me the category ids which was ticked, but I need the full category structure.

Eg :

if a posts ticked only child 2, from above code I get only child 2 cat id. instead of that I need child 2,parent sub 1 & parent 1 cat ids.

How can I do that ?

Answer 1

Have you tried using get_ancestors?

Ideally, you would do this in a hook after the post has been updated and add the parent categories to the post so that you can use them in Queries.