roblib_solr_query_alter/src/EventSubscriber/PostConvertedQueryEventSubscriber.php

<?php

namespace Drupal\roblib_solr_query_alter\EventSubscriber;

use Drupal\advanced_search\AdvancedSearchQuery;
use Drupal\search_api_solr\Event\PostConvertedQueryEvent;
use Drupal\search_api_solr\Event\SearchApiSolrEvents;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * Subscribes to PostConvertedQueryEvents.
 *
 * @package Drupal\advanced_search\EventSubscriber
 */
class PostConvertedQueryEventSubscriber implements EventSubscriberInterface {

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    $events[SearchAPISolrEvents::POST_CONVERT_QUERY][] = ['alterSolariumQuery'];

    return $events;

  }

  /**
   * Alter the query.
   */
  public function alterSolariumQuery(PostConvertedQueryEvent $event) {
    //$search_api_query = $event->getSearchApiQuery();
    $solarium_query = $event->getSolariumQuery();
    $handler = $solarium_query->getHandler();
    if($handler == 'select') {
      $solarium_query->getFilterQuery('index_filter')
        ->setQuery('');
    }
  }

}