Rewrite and custom post type: order by custom field not working

Question

I have a custom post type "placement" with a custom field "position". What I want to do is, on the placements archive page, to order results by the "position" value.

I have the following URL rewrite rule:

$newRules['placements/?$'] = 'index.php?post_type=placement&meta_key=position&orderby=meta_value&order=DESC';

And this is how is defined my custom post type:

    function custom_post_type_placement() {

    $labels = array(
        'name'                => _x( 'Placements', 'Post Type General Name', 'text_domain' ),
        'singular_name'       => _x( 'Placement', 'Post Type Singular Name', 'text_domain' ),
        'menu_name'           => __( 'Placement', 'text_domain' ),
        'name_admin_bar'      => __( 'Placement', 'text_domain' ),
        'parent_item_colon'   => __( 'Parent Placement:', 'text_domain' ),
        'all_items'           => __( 'All Placements', 'text_domain' ),
        'add_new_item'        => __( 'Add New Placement', 'text_domain' ),
        'add_new'             => __( 'Add New', 'text_domain' ),
        'new_item'            => __( 'New Placement', 'text_domain' ),
        'edit_item'           => __( 'Edit Placement', 'text_domain' ),
        'update_item'         => __( 'Update Placement', 'text_domain' ),
        'view_item'           => __( 'View Placement', 'text_domain' ),
        'search_items'        => __( 'Search Placement', 'text_domain' ),
        'not_found'           => __( 'Not found', 'text_domain' ),
        'not_found_in_trash'  => __( 'Not found in Trash', 'text_domain' ),
    );
    $args = array(
        'label'               => __( 'Placement', 'text_domain' ),
        'description'         => __( 'Placements', 'text_domain' ),
        'labels'              => $labels,
        'supports'            => array( 'title', 'editor', ),
        'taxonomies'          => array( 'journal_placement' ),
        'hierarchical'        => false,
        'public'              => true,
        'show_ui'             => true,
        'show_in_menu'        => true,
        'menu_position'       => 5,
        'show_in_admin_bar'   => true,
        'show_in_nav_menus'   => true,
        'can_export'          => true,
        'has_archive'         => true,      
        'exclude_from_search' => false,
        'publicly_queryable'  => true,
        'capability_type'     => 'page',
    );
    register_post_type( 'placement', $args );
add_action( 'init', 'custom_post_type_placement', 0 );

I am able to switch ASC and DESC correctly but my results are still ordered by post date.

Here is what the query looks like:

SELECT SQL_CALC_FOUND_ROWS fetr_posts.ID FROM fetr_posts WHERE 1=1 AND fetr_posts.post_type = 'placement' AND (fetr_posts.post_status = 'publish' OR fetr_posts.post_status = 'private') ORDER BY fetr_posts.post_date DESC LIMIT 0, 10

It seems the meta_key and orderby parameters are not taken into account. Do you have any idea why and how to sort my result properly using a rewrite rule ?

---

EDIT: thanks to Milo's answer, I changed my code the following way and it now works.

I removed completely the rewrite rule and defined my custom post as follow:

function custom_post_type_placement() {

    $labels = array(
        'name'                => _x( 'Placements', 'Post Type General Name', 'text_domain' ),
        'singular_name'       => _x( 'Placement', 'Post Type Singular Name', 'text_domain' ),
        'menu_name'           => __( 'Placement', 'text_domain' ),
        'name_admin_bar'      => __( 'Placement', 'text_domain' ),
        'parent_item_colon'   => __( 'Parent Placement:', 'text_domain' ),
        'all_items'           => __( 'All Placements', 'text_domain' ),
        'add_new_item'        => __( 'Add New Placement', 'text_domain' ),
        'add_new'             => __( 'Add New', 'text_domain' ),
        'new_item'            => __( 'New Placement', 'text_domain' ),
        'edit_item'           => __( 'Edit Placement', 'text_domain' ),
        'update_item'         => __( 'Update Placement', 'text_domain' ),
        'view_item'           => __( 'View Placement', 'text_domain' ),
        'search_items'        => __( 'Search Placement', 'text_domain' ),
        'not_found'           => __( 'Not found', 'text_domain' ),
        'not_found_in_trash'  => __( 'Not found in Trash', 'text_domain' ),
    );
    $rewrite = array(
        'slug'                => 'placements',
        'with_front'          => false,
        'pages'               => true,
        'feeds'               => false,
    );
    $args = array(
        'label'               => __( 'Placement', 'text_domain' ),
        'description'         => __( 'Placements', 'text_domain' ),
        'labels'              => $labels,
        'supports'            => array( 'title', 'editor', ),
        'taxonomies'          => array( 'journal_placement' ),
        'hierarchical'        => false,
        'public'              => true,
        'show_ui'             => true,
        'show_in_menu'        => true,
        'menu_position'       => 5,
        'show_in_admin_bar'   => true,
        'show_in_nav_menus'   => true,
        'can_export'          => true,
        'has_archive'         => true,      
        'exclude_from_search' => false,
        'publicly_queryable'  => true,
        'rewrite'             => $rewrite,
        'capability_type'     => 'page',
    );
    register_post_type( 'placement', $args );

}
add_action( 'init', 'custom_post_type_placement', 0 );

and I also added this function:

function order_by_position($query){

    if(is_post_type_archive( 'placement' )){
        $query->query_vars['meta_key']="position";
        $query->query_vars['orderby']="meta_value_num";
    }
    return $query;
}

add_action( 'pre_get_posts', 'order_by_position' );

Answer 1

If your post type slug is placement, and you set has_archive to true, then the archive WordPress generates is at /placement/. Set has_archive to placements instead and let WordPress add the rewrite rules (which will also handle pagination properly).

Then, to order by your meta key, add a function hooked to pre_get_posts, check if the query is_main_query() and is_post_type_archive( 'placement' ), then set your meta_query parameters there.