Agregar menú de filtro a la lista de publicaciones de administrador (de tipo personalizado) para filtrar publicaciones por valores de campo personalizados

Sencillo de hacer, primero cree el menú desplegable con solo los valores de metadatos que desea y luego capture el envío del filtro, simplemente cambie POST_TYPE al nombre de su tipo de publicación y META_KEY al nombre de su clave de meta :

<?php
/*
Plugin Name: Admin Filter BY Custom Fields
Plugin URI: http://en.bainternet.info
Description: answer to http://wordpress.stackexchange.com/q/45436/2487
Version: 1.0
Author: Bainternet
Author URI: http://en.bainternet.info
*/

add_action( 'restrict_manage_posts', 'wpse45436_admin_posts_filter_restrict_manage_posts' );
/**
 * First create the dropdown
 * make sure to change POST_TYPE to the name of your custom post type
 * 
 * @author Ohad Raz
 * 
 * @return void
 */
function wpse45436_admin_posts_filter_restrict_manage_posts(){
    $type = 'post';
    if (isset($_GET['post_type'])) {
        $type = $_GET['post_type'];
    }

    //only add filter to post type you want
    if ('POST_TYPE' == $type){
        //change this to the list of values you want to show
        //in 'label' => 'value' format
        $values = array(
            'label' => 'value', 
            'label1' => 'value1',
            'label2' => 'value2',
        );
        ?>
        <select name="ADMIN_FILTER_FIELD_VALUE">
        <option value=""><?php _e('Filter By ', 'wose45436'); ?></option>
        <?php
            $current_v = isset($_GET['ADMIN_FILTER_FIELD_VALUE'])? $_GET['ADMIN_FILTER_FIELD_VALUE']:'';
            foreach ($values as $label => $value) {
                printf
                    (
                        '<option value="%s"%s>%s</option>',
                        $value,
                        $value == $current_v? ' selected="selected"':'',
                        $label
                    );
                }
        ?>
        </select>
        <?php
    }
}


add_filter( 'parse_query', 'wpse45436_posts_filter' );
/**
 * if submitted filter by post meta
 * 
 * make sure to change META_KEY to the actual meta key
 * and POST_TYPE to the name of your custom post type
 * @author Ohad Raz
 * @param  (wp_query object) $query
 * 
 * @return Void
 */
function wpse45436_posts_filter( $query ){
    global $pagenow;
    $type = 'post';
    if (isset($_GET['post_type'])) {
        $type = $_GET['post_type'];
    }
    if ( 'POST_TYPE' == $type && is_admin() && $pagenow=='edit.php' && isset($_GET['ADMIN_FILTER_FIELD_VALUE']) && $_GET['ADMIN_FILTER_FIELD_VALUE'] != '') {
        $query->query_vars['meta_key'] = 'META_KEY';
        $query->query_vars['meta_value'] = $_GET['ADMIN_FILTER_FIELD_VALUE'];
    }
}

Si está utilizando otra consulta dentro de restrict_manage_posts , asegúrese de agregar && $query->is_main_query() a su declaración parse_query if, de lo contrario, el filtro parse_query interferirá con esa segunda consulta.

if ( 'POST_TYPE' == $type && is_admin() && $pagenow=='edit.php' && isset($_GET['ADMIN_FILTER_FIELD_VALUE']) && $_GET['ADMIN_FILTER_FIELD_VALUE'] != '' && $query->is_main_query() ) { $query->query_vars['meta_key'] = 'META_KEY'; $query->query_vars['meta_value'] = $_GET['ADMIN_FILTER_FIELD_VALUE']; }