new WP_Query to get max price meta value not working

Question

So, I'm trying to get the maximum price of all 'properties' (custom post type).

$max_price_query = new WP_query(array(
    'posts_per_page' => -1,
    'post_status' => 'publish',
    'post_type' => 'properties',
    'nopaging' => true,
    'orderby' => 'meta_value_num',
    'meta_key' => '_price',
    'order' => 'DESC',
));

if ($max_price_query->have_posts()) {
    while ($max_price_query->have_posts()) : $max_price_query->the_post();
        $pid = $post->ID;
        $price_max = get_post_meta($pid, '_price', true);
        echo '<pre>' . $pid . '</pre>';
    endwhile;
    wp_reset_postdata();
}

First problem is that they are ordered by ID, not price. The properties are called Property 1, Property 2, etc., So in this case, ordered by ID == ordered by Title == ordered by Date.

Second problem, is that, before being ordered by ID, they are actually ordered by another meta_value called _featured_prop (which is either 0 or 1). Well this is because, I also have the following:

 function set_query_parameters($query) {
    if( !is_admin() &&  is_main_query() && is_post_type_archive( 'properties' ) ) {
        $query->set('meta_key', '_featured_prop');
        return $query;
    }
      add_action( 'pre_get_posts', 'set_query_parameters' );

I don't understand why this pre_get_posts seems to be affecting my new WP_Query since I have specified 'is_main_query()' in the if statement. I thought it wouldn't affect my secondary_query.

When echoing the ID's I'm essentially getting: 26, 24, 20, 27, 25, 23, 22, 21, 19. Where 26, 24, and 20, have _featured_prop = 1.

If it makes any difference, the first code snippet is being called in a sidebar.php file which is being called into archive-properties.php. Elsewhere, in a functions file I am calling in function in the second code snippet (since you can't do an add_action of pre_get_posts after pre_get_posts is fired, before loading the archive page).

Can anybody give me some help?

Answer 1

As @Milo suggested and also codex documented it is_main_query Under_the_Hood

This function is an alias for the method WP_Query::is_main_query(). In filter or action hook callbacks that are passed the WP_Query object, such as pre_get_posts, it is circular to call this function. Instead, directly call the passed object's method. For example, if your filter callback assigns the passed WP_Query object to $query, you would call the method like so: $query->is_main_query()

You must change if conduction to

function set_query_parameters($query) {
    if( !is_admin() &&  $query->is_main_query() && is_post_type_archive( 'properties' ) ) {
        $query->set('meta_key', '_featured_prop');
    }
}
add_action( 'pre_get_posts', 'set_query_parameters' );

There is no need to return the query. And I really wonder why you are not receiving a NOTICE because WordPress also trigger _doing_it_wrong() on using is_main_query() in this way!