WP_Query object: what's the difference between [query] and [query_vars]?

Question

I'm debugging a query created by the Post widget of Elementor, it should exclude the post with id 12419. I see the

[post__not_in] => Array
                    (
                        [0] => 12419
                    )

inside the [query] part, but the post is returned by the query (as you can see below from the dump of the query object).

WP_Query Object
(
    [query] => Array
        (
            [posts_per_page] => 20
            [paged] => 1
            [post_status] => publish
            [post_type] => post
            [orderby] => post_date
            [order] => desc
            [ignore_sticky_posts] => 1
            [post__not_in] => Array
                (
                    [0] => 12419
                )
        )
    [query_vars] => Array
        (
            [posts_per_page] => 20
            [paged] => 1
            [post_status] => publish
            [post_type] => post
            [orderby] => post_date
            [order] => DESC
            [ignore_sticky_posts] => 1
            [cache_results] => 1
            [update_post_term_cache] => 1
            [lazy_load_term_meta] => 1
            [update_post_meta_cache] => 1
            [comments_per_page] => 50
        )
    [tax_query] => WP_Tax_Query Object
        (
            [relation] => AND
            [primary_table] => wp_posts
            [primary_id_column] => ID
        )
    [meta_query] => WP_Meta_Query Object
        (
        )
    [request] => SELECT SQL_CALC_FOUND_ROWS  wp_posts.ID FROM wp_posts  WHERE 1=1  AND wp_posts.post_type = 'post' AND ((wp_posts.post_status = 'publish'))  ORDER BY wp_posts.post_date DESC LIMIT 0, 20
    [posts] => Array
        (
            [0] => WP_Post Object
                (
                    [ID] => 12419
                    [post_author] => 7
                    [post_date] => 2021-07-30 14:04:14
                    [post_date_gmt] => 2021-07-30 12:04:14
                    [post_content] => [post_title] => Attacchi di Panico: Cosa Sono, Sintomi, Cause, Conseguenze e Cura
                    [post_status] => publish
                    [comment_status] => open
                    [ping_status] => closed
                    [post_name] => attacchi-di-panico
                    [post_modified] => 2021-07-30 14:04:20
                    [post_modified_gmt] => 2021-07-30 12:04:20
                    [guid] => https://www.psicocultura.it/?p=12419
                    [post_type] => post
                    [filter] => raw
                    [word_count] => 2601
                    [permalink] => https://www.psicocultura.it/attacchi-di-panico/
                )
            [1] => WP_Post Object
                (
                    /*...*/
                )
            
    [current_comment] => -1
    [found_posts] => 137
    [max_num_pages] => 7
    [is_home] => 1
    [query_vars_hash:WP_Query:private] => 4c5b129a335cdd7e2b67ea909788224a
    [query_vars_changed:WP_Query:private] => 1
    [compat_fields:WP_Query:private] => Array
        (
            [0] => query_vars_hash
            [1] => query_vars_changed
        )
    [compat_methods:WP_Query:private] => Array
        (
            [0] => init_query_flags
            [1] => parse_tax_query
        )
)

Answer 1

Those [query] and [query_vars], i.e. WP_Query::$query and WP_Query::$query_vars, are defined as: (see source on Trac)

class WP_Query {

  /**
   * Query vars set by the user
   *
   * @since 1.5.0
   * @var array
   */
  public $query;

  /**
   * Query vars, after parsing
   *
   * @since 1.5.0
   * @var array
   */
  public $query_vars = array();

So the [query] is an array of arguments that are passed to the WP_Query class (the $args array in the following example), whereas the [query_vars] is an array of arguments in the [query] array and the default arguments like the ones listed here and including custom arguments that were not originally set in the [query], but are later set using hooks such as pre_get_posts.

So for example, with this:

$args = array(
    'posts_per_page' => 20,
    'post__not_in'   => [ 12419 ],
    'custom_arg'     => true,
);

$query = new WP_Query( $args );

• var_dump( $query->query ); would output only the items in the above array:

  array(3) {
    ["posts_per_page"]=>
    int(20)
    ["post__not_in"]=>
    array(1) {
      [0]=>
      int(12419)
    }
    ["custom_arg"]=>
    bool(true)
  }
  

• var_dump( $query->query_vars ); would also output the same items, but with many other items (note the array(64) below) like error and m:

  array(64) {
    ["posts_per_page"]=>
    int(20)
    ["post__not_in"]=>
    array(1) {
      [0]=>
      int(12419)
    }
    ["custom_arg"]=>
    bool(true)
    ["error"]=>
    string(0) ""
    ["m"]=>
    string(0) ""
    ["p"]=>
    int(0)
    ...
  }
  

Now, if I used pre_get_posts to change the post__not_in argument and also to add another custom argument:

add_action( 'pre_get_posts', function ( $query ) {
    if ( $query->get( 'custom_arg' ) ) {
        $query->set( 'post__not_in', [] );

        $query->set( 'custom_arg2', 'foo bar baz' );
    }
} );

• var_dump( $query->query ); would show no changes, but var_dump( $query->query_vars ); would now output:

  array(65) {
    ["posts_per_page"]=>
    int(20)
    ["post__not_in"]=>
    array(0) {
    }
    ["custom_arg"]=>
    bool(true)
    ["error"]=>
    string(0) ""
    ["m"]=>
    string(0) ""
    ["p"]=>
    int(0)
    ...
    ["custom_arg2"]=>
    string(11) "foo bar baz"
    ...
  }
  

• And if I apply array_filter() on the query_vars array, i.e. var_dump( array_filter( $query->query_vars ) ), then the output would no longer contain the post__not_in:

  array(9) {
    ["posts_per_page"]=>
    int(20)
    ["custom_arg"]=>
    bool(true)
    ["custom_arg2"]=>
    string(11) "foo bar baz"
    ["cache_results"]=>
    bool(true)
    ["update_post_term_cache"]=>
    bool(true)
    ["lazy_load_term_meta"]=>
    bool(true)
    ["update_post_meta_cache"]=>
    bool(true)
    ["comments_per_page"]=>
    string(2) "50"
    ["order"]=>
    string(4) "DESC"
  }
  

So I don't know how did you debug the query in question, i.e. I don't know what code or plugin you used, but I presumed that the [query_vars] values were filtered (empty ones were removed) (by your debug code?) and secondly, the post__not_in argument is probably being filtered similar or the same way I did it above.

Therefore if you can, try suppressing filters by adding 'suppress_filters' => true to the query arguments, and see if the post__not_in array is now good or that the post 12419 is now being excluded from the results.