Filter in Custom post type to find the parent post

Question

function fws_admin_posts_filter( $query ) {
global $pagenow;
if ( is_admin() && $pagenow == 'edit.php' && !empty($_GET['my_parent_pages'])) {
    $query->query_vars['post_parent'] = $_GET['my_parent_pages'];
}
}

add_filter( 'parse_query', 'fws_admin_posts_filter' );

function admin_page_filter_parentpages() {
global $wpdb;
if (isset($_GET['post_type']) && $_GET['post_type'] == 'books') {
    $sql = "SELECT ID, post_title FROM ".$wpdb->posts." WHERE post_type = 'books' AND post_parent = 0 AND post_status = 'publish' ORDER BY post_title";
    $parent_pages = $wpdb->get_results($sql, OBJECT_K);
    $select = '
        <select name="my_parent_pages">
            <option value="">Parent Pages</option>';
    $current = isset($_GET['my_parent_pages']) ? $_GET['my_parent_pages'] : '';
    foreach ($parent_pages as $page) {
        $select .= sprintf('
            <option value="%s"%s>%s</option>', $page->ID, $page->ID == $current ? ' selected="selected"' : '', $page->post_title);
    }
    $select .= '
        </select>';
    echo $select;
} else {
    return;
   }
 }
 add_action( 'restrict_manage_posts', 'admin_page_filter_parentpages' );

This code gives me the expended filter dropdown option but once I select one of the option and hit filter then the return page is empty.

Once I select one of them and hit filter, the result page can't bring back the result.

Answer 1

Let's if it helps you. Instead of adding a custom query var if we can use directly from url that might solve the issu. First make post_parent loadable from url.

function make_post_parent_public_qv() {
    global $pagenow;
    if ( is_admin() && $pagenow == 'edit.php' )
        $GLOBALS['wp']->add_query_var( 'post_parent' );
}
add_action( 'init', 'make_post_parent_public_qv' );

Now focus on the filter:

function admin_page_filter_parentpages() {
    global $wpdb;
    if (isset($_GET['post_type']) && $_GET['post_type'] == 'books') {
        $sql = "SELECT ID, post_title FROM ".$wpdb->posts." WHERE post_type = 'books' AND post_parent = 0 AND post_status = 'publish' ORDER BY post_title";
        $parent_pages = $wpdb->get_results($sql, OBJECT_K);
        $select = '
            <select name="post_parent">
                <option value="">Parent Pages</option>';
                $current = isset($_GET['post_parent']) ? $_GET['post_parent'] : '';
                foreach ($parent_pages as $page) {
                    $select .= sprintf('<option value="%s"%s>%s</option>', $page->ID, $page->ID == $current ? ' selected="selected"' : '', $page->post_title);
                }
        $select .= '
            </select>';
        echo $select;
    } else {
        return;
   }
}
add_action( 'restrict_manage_posts', 'admin_page_filter_parentpages' );

Give it a try and let me know.