Add option to sort posts by a number field

[andergmartins]

I have implemented a workaround for a user:

<?php
/**
 * @param WP_Query $query
 *
 * @return bool
 */
function pp_should_change_post_order($query)
{
    // Add a conditional here to filter when the queries should be filtered.
    return true;
}

add_filter('posts_fields', 'pp_order_posts_by_metadata_select', 10, 2);
function pp_order_posts_by_metadata_select($fields, $query)
{
    if (!pp_should_change_post_order($query)) {
        return $fields;
    }

    $fields .= ', IF(pm.meta_value > 0, pm.meta_value, 999999) AS cal_custom_position';

    return $fields;
}

add_filter('posts_join', 'pp_order_posts_by_metadata_join', 10, 2);
function pp_order_posts_by_metadata_join($join, $query)
{
    if (!pp_should_change_post_order($query)) {
        return $join;
    }

    global $wpdb;
    $join = "LEFT JOIN {$wpdb->postmeta} as pm ON (ID = pm.post_id AND pm.meta_key = '_pp_editorial_meta_number_position')";

    return $join;
}

add_filter('posts_orderby', 'pp_order_posts_by_metadata', 10, 2);
function pp_order_posts_by_metadata($orderby, $query)
{
    if (!pp_should_change_post_order($query)) {
        return $orderby;
    }

    $originalOrderby = $orderby;

    $orderby = 'cal_custom_position ASC';

    if (!empty($originalOrderby)) {
        $orderby .= ',' . $originalOrderby;
    }

    return $orderby;
}

Which sorts the posts by an editorial metadata field called "position". This is not the best solution because it forces to join the metadata table on every post request.

Maybe we could use the field "menu_order" for that?

It can be enabled for posts using :

add_action( 'admin_init', 'pp_add_page_attributes' );

function pp_add_page_attributes() 
{
    add_post_type_support( 'post', 'page-attributes' );
}

But this is not working for Gutenberg, which changes the field value to 0 on every post save.
But we could map a special custom field to that post field and display it as editorial metadata.
We should allow only one field of that type, until we don't provide custom set of fields based on post type or categories.