Post edit.php – add custom field to filter section

UPDATE: I was able to find a solution but it displays all the custom fields. I only want it to display one custom field. I’m not familiar with PHP, so all help is greatly appreciated.

add_filter( 'parse_query', 'ba_admin_posts_filter' );
add_action( 'restrict_manage_posts', 'ba_admin_posts_filter_restrict_manage_posts' );
function ba_admin_posts_filter( $query )
{
    global $pagenow;
    if ( is_admin() && $pagenow=='edit.php' && isset($_GET['ADMIN_FILTER_FIELD_NAME']) && $_GET['ADMIN_FILTER_FIELD_NAME'] != '') {
        $query->query_vars['meta_key'] = $_GET['ADMIN_FILTER_FIELD_NAME'];
    if (isset($_GET['ADMIN_FILTER_FIELD_VALUE']) && $_GET['ADMIN_FILTER_FIELD_VALUE'] != '')
        $query->query_vars['meta_value'] = $_GET['ADMIN_FILTER_FIELD_VALUE'];
    }
}
function ba_admin_posts_filter_restrict_manage_posts()
{
    global $wpdb;
    $sql = 'SELECT DISTINCT meta_key FROM '.$wpdb->postmeta.' ORDER BY 1';
    $fields = $wpdb->get_results($sql, ARRAY_N);
?>
<select name="ADMIN_FILTER_FIELD_NAME">
<option value=""><?php _e('Filter By Custom Fields', 'baapf'); ?></option>
<?php
    $current = isset($_GET['ADMIN_FILTER_FIELD_NAME'])? $_GET['ADMIN_FILTER_FIELD_NAME']:'';
    $current_v = isset($_GET['ADMIN_FILTER_FIELD_VALUE'])? $_GET['ADMIN_FILTER_FIELD_VALUE']:'';
    foreach ($fields as $field) {
        if (substr($field[0],0,1) != "_"){
        printf
            (
                '<option value="%s"%s>%s</option>',
                $field[0],
                $field[0] == $current? ' selected="selected"':'',
                $field[0]
            );
        }
    }
?>
</select>
<?php
}
?>

Oh well, I can leave it like the above.

Hi There,

I came across your post and was using the same code. I needed to display a drop down with only one custom key.

To do that I replaced

$sql = 'SELECT DISTINCT meta_key FROM '.$wpdb->postmeta.' ORDER BY 1';
to
$sql= "SELECT DISTINCT meta_key FROM $wpdb->postmeta WHERE meta_key = 'product_code_box_text' ORDER BY 1";

Hope that helps even now:)