pre_get_post filter returns results when there should not be

Question

I am trying to filter a CPT by an array of post ids using pre_get_post filter within my plugin. Everything works fine when there are post that match the ids, but when there are no post that match the ids I am getting all the post within the CPT that I am trying to filter in return. What I need id there are not post ids found (or post from the post ids) is the archive page to display the normal 'no results found'.

This is a run down of my script...

add_action('pre_get_posts', array(__CLASS__, 'get_posts')); //run inside a class
function get_posts($wp_query) {

        if ($wp_query->is_main_query() && !is_admin() && is_post_type_archive(MbeEnrollments::MBE_POST_TYPE_NAME) && $wp_query->query_vars['list-view'] == 'list-view') {

      //For the purpose of this sample I removed the script in the section that produces the CPT post ids that I am trying to return. This portion works fine and as expected.

            //if post ids are found 
            if ($post_ids) {

                $wp_query->set('post__in', $post_ids);

            }
            return;


        }


    }

I assume the issue is that the CPT archive page will run as normal if my if($post_ids) clause return false. However, I am trying to find a way to make it return with no results. Thanks in advance for any help.

UPDATE: If I include a post id that I know does not exist within the CPT I get the desired 'No Results Found'. Even though this works it seems a little bit like a hack and there has to be a cleaner way. Any ideas? Here is the if that works....

            if ($post_ids) { 

                $wp_query->set('post__in', $post_ids);

            }else{//if no post ids then pass in a fake post id to get the 'no results found'

                $wp_query->set('post__in', array(0));

            }
            return;

Answer 1

Assuming that you get an empty array for $post_ids if there are no matching posts, I think this is wrong:

if ( $post_ids ) {}

Because the array exists, but is empty. Try to check for an empty array:

if ( ! empty( $post_ids ) ) {}

Answer 2

$query->set( 'post__in', $post_ids );

This line is responsible to apply filters on the query.

But you are using it ONLY when $post_ids is true. that means that when $post_ids evaluates to false, then you are not filtering.

Basically, you are saying:

if ($post_ids) { //if there are values to filter

                $wp_query->set('post__in', $post_ids); // then use them to filter

            } // otherwise don't filter.
            return;

So just make sure that $post_ids is always an array, even an empty one. Then don't conditionnally set the query.