Remove the Parent category from the permalink but leave the child category

Question

I had a site with a url structure like

example.com/news/story-1
example.com/news/local/story-2
example.com/food/tacos/story-3

Now it's been moves to a multisite, with /news/ and /food/ as the site subfolders.

Within those sub-sites, the top level category is News or Food, respectively. So if the permalink is set to /%category%/%postname%/, the permalink come out as

example.com/news/news/local/story-title
example.com/food/food/tacos/story-title

In order to remove the category that's a duplicate of the site subfolder, I added this to functions.php:

add_filter( 'post_link', 'remove_parent_category', 10, 3 );
    function remove_parent_category( $permalink, $post, $leavename ) {
        $permalink_array = explode("/", $permalink);
        $clean_permalink = array_unique($permalink_array);
        $new_permalink = implode("/", $clean_permalink);
        return $new_permalink;
    }  

This works, giving me back

example.com/news/local/story-2
example.com/food/tacos/story-3

and the ability to navigate to

example.com/news/
example.com/news/local
etc.

However posts that are in the top level category now 404 (which I suppose is to be expected) because their url is now

example.com/news/story-1

(which is what I want).

From what I'm gathering I think I need to use either pre_post_link or a rewrite rule to treat those top level posts as if my permalink structure is just %postname%/, but that's where I'm stuck.

UPDATE: Solved (I think?)

T.Todua's answer below got me started:

• posts in child categories work
• category and sub-category archives work
• pages work

but posts that had their last category stripped (posts in the top-most category) were still 404ing.

After much poking around with WP Rewrite and discovering that the way I thought I'd be able to solve this would not work, it did lead me down another path.

It turns out these "naked" posts with no category were hitting the category rule in WP Rewrite — so the query ended up being category_name = this-is-the-slug-for-a-post. There was no way (at least that I could figure out) to prevent this in WP Rewrite, since there's no way to tell the difference between a category slug and a post slug using RegEx.

(Though I am wondering how, if your permalink structure is just %postname%/, Wordpress differentiates between a page and a post. If anyone knows, I'd be interested.)

From there I found this post and modified it to my needs. It catches category requests and checks if there's a real category with that name. If not, it converts it to a post query.

The whole thing is below: please let me know if you see any major red flags or problems down the road with this approach.

add_filter('post_link','remove_parent_category', 10, 3 );
add_filter('post_type_link','remove_parent_category', 10, 3 ); 
add_filter('category_link','remove_parent_category', 10, 2);

function remove_parent_category($termlink, $term_id )
{
    return implode("/", array_unique(explode("/", $termlink)));
}

add_filter('request', function(array $query_vars) {
    // do nothing in wp-admin
    if(is_admin()) {
        return $query_vars;
    }
    // if the query is for a category
    if(isset($query_vars['category_name'])) {
        $query_cat = $query_vars['category_name'];
        // if it's a real category do nothing
        if(term_exists($query_cat, "category")) {
            return $query_vars;
        };
        // save the slug
        $postname = $query_vars['category_name'];
        // completely replace the query with a post query
        $query_vars = array('name' => "$postname");
    }
    return $query_vars;
});

Answer 1

I think, there are two things to be resolved:

1) You need that posts (and other post types) also followed that command, so use both:

add_filter( 'post_link',      'remove_parent_category', 10, 3 );
add_filter( 'post_type_link', 'remove_parent_category', 10, 3 ); 

2) but then categories also need to have changed url, so use:

add_filter( 'category_link', 'remove_parent_category_2', 10, 2);
function remove_parent_category_2($termlink, $term_id )
{
    return implode("/", array_unique(explode("/", $termlink)));
}

after that, go to SETTINGS > PERMALINKS > SAVE and see if it works.

btw, what I suggest, is (after backup of database) just to remove those categories from categories at all (from WP dashboard > categories), and WP will do everything itself, without need for custom codes.