How set a default value for search results?

@Giuseppe87 how did you override the default querystring plugin for facets to use the above code? Is there something in a services.yml file somewhere? I didn't see anything in the Facet config to select the new processor.

Oh, nevermind, found the answer. For others: Go to the Configure link for your Facet Source and there will be a radio selector for the url processor.

Thanks a lot for sharing this Giuseppe87!
Creating a custom URL Processor in order to pre-fill / force a value on a facet served for us as a workaround to the facet + contextual filter problem.
Indeed, on each node of type School, we are displaying the list of all trainings related to that specific School.
At first, we obviously used Views' Contextual filter for that.
However, we also had some facet filters with search API for the Level of the training for example.
Unfortunately, facets are not aware of the data transmitted to the view through contextual filters. So they were not properly restricting the available options regarding to the currently viewed School page.
In the end, we removed the contextual filter and replaced it with an additional facet "School" which value is pre-filled / forced with the current School (retrieved form canonical url) in a custom URL Processor, following code at #2.

#2 Thanks for your solution but it works when we have only one search page. In another case we'll apply the default value for all pages as alterRequest() will be called each time where we're rendering facet even when the facet related to default value is not expected to be displayed.
I got the same result with the next code:

<?php

namespace Drupal\my_module\Plugin\facets\url_processor;

use Drupal\facets\Plugin\facets\url_processor\QueryString;
use Drupal\facets\FacetInterface;

/**
 * Query string URL processor.
 *
 * @FacetsUrlProcessor(
 *   id = "my_query_string",
 *   label = @Translation("My Query string"),
 *   description = @Translation("Add description")
 * )
 */
class MyQueryString extends QueryString {

  /**
   * {@inheritdoc}
   */
  public function buildUrls(FacetInterface $facet, array $results) {
    if ($facet->getOriginalId() == 'content_type') {
      if (!$facet->getActiveItems()) {
        $facet->setActiveItem('prodotto');
      }
    }
    return parent::buildUrls($facet, $results);
  }

}

mb it'll be helpful for someone

#7 did not work for me. Despite the facet had a default value, the results were not filtered.
#2 worked for me. The facet had a default value as well the results were filtered.

Should this perhaps be changed to a feature request to allow defining default values in facets? Might be a relevant feature?

We should ensure that not only a single value, but always a list of values is allowed to be defined per facet.

Approach #7 allows me to set default filter value but please dont forget to set processor for the facet source configuration.

For me,

#7 sets the default facet value in the facet field, but the results are not actually filtered accordingly.

#2 works for me. However, since then a new argument ($urlGenerator) was added to the url_processor constructor (the one we are extending), so I had to add it here as well, as follows:

<?php

namespace Drupal\my_module\Plugin\facets\url_processor;

use Drupal\Core\Entity\EntityTypeManagerInterface;
use Drupal\facets\Plugin\facets\url_processor\QueryString;
use Drupal\facets\Utility\FacetsUrlGenerator;
use Symfony\Component\EventDispatcher\EventDispatcherInterface;
use Symfony\Component\HttpFoundation\Request;

/**
 * Query string URL processor.
 *
 * @FacetsUrlProcessor(
 *   id = "my_query_string",
 *   label = @Translation("My Query string"),
 *   description = @Translation("Add description")
 * )
 */
class MyQueryString extends QueryString {

  /**
   * A string of how to represent the facet in the url.
   *
   * @var string
   */
  protected $urlAlias;

  /**
   * The event dispatcher.
   *
   * @var \Symfony\Component\EventDispatcher\EventDispatcherInterface
   */
  protected $eventDispatcher;

  /**
   * {@inheritdoc}
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, Request $request, EntityTypeManagerInterface $entity_type_manager, EventDispatcherInterface $eventDispatcher, FacetsUrlGenerator $urlGenerator) {
    $this->alterRequest($request);
    parent::__construct($configuration, $plugin_id, $plugin_definition, $request, $entity_type_manager, $eventDispatcher, $urlGenerator);
  }

  /**
   * @param \Symfony\Component\HttpFoundation\Request $request
   */
  private function alterRequest(Request $request): void {
    $query = $request->query;
    $facet_new_parameters = [];

    if ($facet_parameters = $query->get('f')) {
      $article_type_filter_exists = FALSE;

      foreach ($facet_parameters as $parameter) {
        $sepator = ':';
        $explosion = explode($sepator, $parameter);
        $facet_type = array_shift($explosion);
        if ($facet_type === 'article_type') {
          $article_type_filter_exists = TRUE;
        }
      }

      if (!$article_type_filter_exists) {
        $facet_new_parameters = array_merge($facet_parameters, ['article_type:10372']);
      }
    }
    else {
      $facet_new_parameters = ['article_type:10372'];
    }

    if (!empty($facet_new_parameters)) {
      $query->set('f', $facet_new_parameters);
    }
  }
}

That said, I agree with #9 -- this should be something that we can set in the facet config.

Thanks all!

$query->get('f') will not work anymore since it's not a scalar value but an array
I used $query->all()['f'] ?? [] instead

problem with this approach is that you can't even intentionally reset the filter to "any" since then the value is not in the URL and it will force the default

Based on the previous comments of this thread, I was able to build the following URL processor.

In my case, I wanted that in some views only the values from user fields became the default values for facets when entering the page. In some other views though, the default query_string plugin is what I needed. Read the inline comments if you intend to use it. Might require some refactoring for your case.

namespace Drupal\mymodule\Plugin\facets\url_processor;

use Drupal\Core\Cache\UnchangingCacheableDependencyTrait;
use Drupal\Core\Entity\EntityTypeManagerInterface;
use Drupal\facets\Plugin\facets\url_processor\QueryString;
use Drupal\facets\Utility\FacetsUrlGenerator;
use Symfony\Component\EventDispatcher\EventDispatcherInterface;
use Symfony\Component\HttpFoundation\Request;

/**
 * Query string URL processor.
 *
 * @FacetsUrlProcessor(
 *   id = "mymodule_query_string",
 *   label = @Translation("My module query string"),
 *   description = @Translation("Extends the original query string with predefined values")
 * )
 */
class MymoduleQueryString extends QueryString {

  use UnchangingCacheableDependencyTrait;

  /**
   * {@inheritdoc}
   *
   * Since we are operating on the construct function and this gets called for
   * every facet on every page, we have to tamper the query object and then
   * restore it; otherwise facets without default values don't seem to work.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, Request $request, EntityTypeManagerInterface $entity_type_manager, EventDispatcherInterface $eventDispatcher, FacetsUrlGenerator $urlGenerator) {
    // Keep a clone of the original query to restore it later.
    $query = clone $request->query;
    /** @var \Drupal\facets\Entity\Facet $facet */
    $facet = $configuration['facet'];
    if ($facet->getFacetSourceConfig()->getUrlProcessorName() === $plugin_id) {
      $this->alterRequest($request);
    }
    parent::__construct($configuration, $plugin_id, $plugin_definition, $request, $entity_type_manager, $eventDispatcher, $urlGenerator);
    if ($facet->getFacetSourceConfig()->getUrlProcessorName() === $plugin_id) {
      // Restore the original query.
      $request->query = $query;
    }
  }

  /**
   * Alter the request object used by facets.
   *
   * When this function gets called, the default constructor has not yet been
   * called; thus we have to take all the variables we need as arguments instead
   * of relying on $this.
   *
   * @param \Symfony\Component\HttpFoundation\Request $request
   *   The current request object.
   */
  protected function alterRequest(Request $request) {
    // For this static variable to have any performance improvements, you must
    // have big_pipe module turned off. Otherwise, you would require another way
    // of caching due to different requests; maybe using a computed field on the
    // user entity is a possible solution.
    static $facet_parameters = [];
    /** @var \Drupal\user\UserInterface $account */
    $account = \Drupal::entityTypeManager()->getStorage('user')->load(\Drupal::currentUser()->id());
    $query = &$request->query;
    // Allow "?disable_defaults=1" for a "Clear defaults" button to be able to
    // work at this point.
    if ($query->getInt('disable_defaults', 0)) {
      return;
    }

    if (empty($facet_parameters)) {
      $original_query_parameters = $query->all()['f'] ?? [];
      if (empty($original_query_parameters)) {
        $facet_parameters = [];

        // Get user fields...
        if ($account->hasField('field_name_1') && !$account->get('field_name_1')->isEmpty()) {
          foreach ($account->get('field_name_1')->getValue() as $values) {
            $facet_parameters[] = 'facet_alias1:' . $values['target_id'];
          }
        }
        if ($account->hasField('field_name_2') && !$account->get('field_name_2')->isEmpty()) {
          foreach ($account->get('field_name_2')->getValue() as $values) {
            $facet_parameters[] = 'facet_alias2:' . $values['target_id'];
          }
        }
      }
    }

    if (!empty($facet_parameters)) {
      $query->set('f', $facet_parameters);
    }
  }

}

It seems, my custom URL processor is being called event when the facet source is set to use the default one.

Anyone encountered this issue?

EDIT: nevermind, it seems another facet source is being triggered on the same page, I just have to find out why.