Add a way easily perform advanced post fields queries

[thelevicole]

Currently if you want to perform advanced queries on the posts table you have to add your custom SQL to the query with the posts_where filter.

For example, let's say we want to get the next page with a menu_order value greater than the current page we would currently do something like:

<?php
add_filter( 'posts_where', function( $where, $query ) {
    global $wpdb;

    $value = $query->query['menu_order_gt'] ?? null;

    if ( is_numeric( $value ) ) {   
        $where .= " AND {$wpdb->posts}.menu_order > {$value} ";
    }

    return $where;
}, 10, 2 );

$query = new WP_Query( [
  'post_type' => 'page',
  'posts_per_page' => 1,
  'menu_order_gt' => (int)get_post_field( 'menu_order' )
] );

I suggest adding a class like WP_Meta_Query but for post fields meaning we could achieve the same result as above but in a much cleaner and simpler way...

<?php
$query = new WP_Query( [
  'post_type' => 'page',
  'posts_per_page' => 1,
  'field_query' => [
    [
      'field' => 'menu_order',
      'value' => (int)get_post_field( 'menu_order' ),
      'compare' => '>',
      'type' => 'NUMERIC'
    ]
  ]
] );