Faking "Same Slug Root, Multiple Custom Post Types" with Redirects?

Question

I know you can't have:

/my-cpt1/name-of-post-1/

and

/my-cpt2/name-of-post-2/

both live at:

/my-ideal-root/name-of-post-1/
/my-ideal-root/name-of-post-2/

But I have a situation where I have content that must be differentiated into different custom post types (because of where it is used elsewhere), that I would like to also be represented on the front end as living under one root path, let's say that's /rules/. Let's also say that I can guarantee /rules/ won't be occupied by anything else (that is, I won't put a page there or any other content under it).

Is it possible to somehow redirect:

/my-cpt1/name-of-post-a/
/my-cpt2/name-of-post-b/
/my-cpt3/name-of-post-c/
...

and so on to:

/potato/name-of-post-a/
/potato/name-of-post-b/
/potato/name-of-post-c/

Maybe after each CPT resolves to its default URL /my-cpt-1/whatever/, I can redirect it to the /rules/my-cpt-1/whatever, and render content in a template at that path, pulling the data based on /whatever/ from the correct CPT (/my-cpt-1/) in the path?

Answer 1

You can have two CPT under a common "prefix"

/potato/name-of-post-from-CPT_1
/potato/name-of-post-from-CPT_2

but you need to create custom rewrite rules and change links generated by WP for these CPTs.

Links created for CPT can be changed with post_type_link filter. Before changing the link, check what format is in use, "plain" (http://example.com/?p=N) or "pretty permalinks" (http://example.com/page-name/).

add_filter( 'post_type_link', 'se390940_oneslug_permalinks', 10, 3 );

function se390940_permalinks( $permalink, $post, $leavename )
{
    global $wp_rewrite;

    if ( !$wp_rewrite->using_permalinks() )
        return $permalink;
    //
    // apply changes to my CPTs only
    $cpts = ['my-cpt1', 'my-cpt2'];
    if ( !in_array( $post->post_type, $cpts ) )
        return $permalink;

    return home_url( "/potato/{$post->post_name}/" );
}

Now you need to add rewrite rules to recognize new links and display relevant posts.
This fragment post_type[]=my-cpt1&post_type[]=my-cpt2 means that post_type is an array with two values and causes post search in both types.

add_action( 'init', 'se390940_oneslug_rewrite' );

function se390940_oneslug_rewrite()
{
    // archive page
    add_rewrite_rule(
        'potato(?:/([0-9]+))?/?$',
        'index.php?post_type[]=my-cpt1&post_type[]=my-cpt2&paged=$matches[1]',
        'top'
    );
    // CPT single page
    add_rewrite_rule(
        'potato/([^/]+)(?:/([0-9]+))?/?$',
        'index.php?post_type[]=my-cpt1&post_type[]=my-cpt2&name=$matches[1]&page=$matches[2]',
        'top'
    );
}

To make rewrite rules work, you have to click Save in Dashboard -> Settings -> Permalinks or use flush_rewrite_fules() when switching the theme ("important note")