Is instantiating WP_Query not possible within an admin Ajax call?

Question

While using the WordPress Ajax API to query certain posts the content of WP_Query is always empty (that means the class exists but every property is set to null or false).

function my_ajax_admin() {
        $query = new WP_Query();
        $tab = array();


        while($query->have_posts()):
            // never reached
        endwhile;
        die(json_encode($out));
    }
}

Is it in general not possible to use WP_Query in Ajax called? May I load something into the function to make it work? Do I require additional parameters?

Answer 1

Note that the constructor of WP_Query is:

public function __construct( $query = '' ) {
    if ( ! empty( $query ) ) {
        $this->query( $query );
    }
}

so you need a non-empty query input in your current code snippet.

Here's a simple test example:

$query = new WP_Query( ['post_type' => 'post' ] );
wp_send_json_success( $query->posts );

but you can check out e.g. wp_ajax_query_attachments() to see how WordPress uses WP_Query in an ajax call.

Also check out the built-in Rest API, it might already have the query you're looking for?