Permalink with Taxonomies in Wordpress in the form /%term_id%/%term_slug%

Question

I need a permalink to be in the form /writer/%term_id%/%term_slug/, as /writer/123/hand-made-shoes

I get the following code but it do not resolve the correct permalink. What could I be missing?

function writer_structure(){
    global $wp_rewrite;

    $writer_structure = '/writers/%writer_id%/%writer%';
    $wp_rewrite->add_rewrite_tag('%writer%', '([^/]+)', "writer=");
    $wp_rewrite->add_permastruct('writer', $writer_structure, false);
}

function writer_permalink($permalink, $post_id, $leavename){
    if (get_option('permalink_structure') != ''){
        $post = get_post($post_id);
        $rewritecode = array(
            '%writer_id%',
            '%writer%'
        );
        if (strpos($permalink, '%writer%') !== FALSE){   
            $terms = wp_get_object_terms($post->ID, 'writer');  
            if (!is_wp_error($terms) && !empty($terms) && is_object($terms[0])) $writer = $terms[0]->slug;
            else $writer = '';
        }
        $rewritereplace = array(
            $post->ID,
            $writer
        );
        $permalink = str_replace($rewritecode, $rewritereplace, $permalink);
    } 
    return $permalink;
}

Thanks.