Removing ?post_type=foo from the url for custom taxonomies

If you create a custom taxonomy which applies to precisely one custom post type, you can bypass the need for ?post_type=foo in your permalinks using the following code.

if (!function_exists('chmac_pre_get_posts')):
function chmac_pre_get_posts($query) {
    if (is_main_query() && !is_admin() && !empty($query->query_vars['CUSTOM_TAXONOMY']))
        $query->query_vars['post_type'] = $query->query['post_type'] = 'CUSTOM_POST_TYPE';
}
endif;
add_filter('pre_get_posts', 'chmac_pre_get_posts');

I spent an hour or two looking for this and ended up writing the code in a few minutes. Hopefully this saves somebody else the hassle.

Love & joy – Callum.

PS> Naturally you need to replace CUSTOM_TAXONOMY and CUSTOM_POST_TYPE with appropriate values (in lowercase).