Issue #2898903 by tetranz, alexpott, smustgrave, immaculatexavier,...

    $taxonomy_storage = $this->entityTypeManager->getStorage('taxonomy_term');

    $vocabulary = $vocab_storage->load($term->bundle());

    $parent = array_keys($taxonomy_storage->loadParents($term->id()));

    $parent = [];

    // Get the parent directly from the term as

    // \Drupal\taxonomy\TermStorageInterface::loadParents() excludes the root.

    foreach ($term->get('parent') as $item) {

      $parent[] = (int) $item->target_id;

    }

    $form_state->set(['taxonomy', 'parent'], $parent);

    $form_state->set(['taxonomy', 'vocabulary'], $vocabulary);

    if (!$this->config('taxonomy.settings')->get('override_selector')) {

      $exclude = [];

      if (!$term->isNew()) {

        $parent = array_keys($taxonomy_storage->loadParents($term->id()));

        $children = $taxonomy_storage->loadTree($vocabulary->id(), $term->id());

        // A term can't be the child of itself, nor of its children.

use Drupal\Core\Field\FieldStorageDefinitionInterface;

use Drupal\field\Entity\FieldConfig;

use Drupal\taxonomy\Entity\Term;

use Drupal\taxonomy\TermInterface;

use Drupal\Tests\system\Functional\Menu\AssertBreadcrumbTrait;

/**

   * Tests saving a term with multiple parents through the UI.

   */

  public function testTermMultipleParentsInterface() {

    // Add a new term to the vocabulary so that we can have multiple parents.

    $parent = $this->createTerm($this->vocabulary);

    // Add two new terms to the vocabulary so that we can have multiple parents.

    // These will be terms with tids of 1 and 2 respectively.

    $this->createTerm($this->vocabulary);

    $this->createTerm($this->vocabulary);

    // Add a new term with multiple parents.

    $edit = [

      'name[0][value]' => $this->randomMachineName(12),

      'description[0][value]' => $this->randomMachineName(100),

      'parent[]' => [0, $parent->id()],

      'parent[]' => [0, 1],

    ];

    // Save the new term.

    $this->drupalGet('admin/structure/taxonomy/manage/' . $this->vocabulary->id() . '/add');

    $this->submitForm($edit, 'Save');

    // Check that the term was successfully created.

    $terms = \Drupal::entityTypeManager()->getStorage('taxonomy_term')->loadByProperties([

      'name' => $edit['name[0][value]'],

    ]);

    $term = reset($terms);

    $term = $this->reloadTermByName($edit['name[0][value]']);

    $this->assertNotNull($term, 'Term found in database.');

    $this->assertEquals($edit['name[0][value]'], $term->getName(), 'Term name was successfully saved.');

    $this->assertEquals($edit['description[0][value]'], $term->getDescription(), 'Term description was successfully saved.');

    // Check that the parent tid is still there. The other parent (<root>) is

    // not added by \Drupal\taxonomy\TermStorageInterface::loadParents().

    $parents = $this->container->get('entity_type.manager')->getStorage('taxonomy_term')->loadParents($term->id());

    $parent = reset($parents);

    $this->assertEquals($edit['parent[]'][1], $parent->id(), 'Term parents were successfully saved.');

    // Check that we have the expected parents.

    $this->assertEquals([0, 1], $this->getParentTids($term), 'Term parents (root plus one) were successfully saved.');

    // Load the edit form and save again to ensure parent are preserved.

    // Generate a new name, so we know that the term really is saved.

    $this->drupalGet('taxonomy/term/' . $term->id() . '/edit');

    $edit = [

      'name[0][value]' => $this->randomMachineName(12),

    ];

    $this->submitForm($edit, 'Save');

    $this->submitForm([], 'Save');

    // Check that we still have the expected parents.

    $term = $this->reloadTermByName($edit['name[0][value]']);

    $this->assertEquals([0, 1], $this->getParentTids($term), 'Term parents (root plus one) were successfully saved again.');

    // Save with two real parents. i.e., not including <root>.

    $this->drupalGet('taxonomy/term/' . $term->id() . '/edit');

    $edit = [

      'name[0][value]' => $this->randomMachineName(12),

      'parent[]' => [1, 2],

    ];

    $this->submitForm($edit, 'Save');

    $this->submitForm([], 'Save');

    // Check that we have the expected parents.

    $term = $this->reloadTermByName($edit['name[0][value]']);

    $this->assertEquals([1, 2], $this->getParentTids($term), 'Term parents (two real) were successfully saved.');

  }

  /**

   * Reloads a term by name.

   *

   * @param string $name

   *   The name of the term.

   *

   * @return \Drupal\taxonomy\TermInterface

   *   The reloaded term.

   */

  private function reloadTermByName(string $name): TermInterface {

    \Drupal::entityTypeManager()->getStorage('taxonomy_term')->resetCache();

    /** @var \Drupal\taxonomy\TermInterface[] $terms */

    $terms = \Drupal::entityTypeManager()->getStorage('taxonomy_term')->loadByProperties(['name' => $name]);

    return reset($terms);

  }

  /**

   * Get the parent tids for a term including root.

   *

   * @param \Drupal\taxonomy\TermInterface $term

   *   The term.

   *

   * @return array

   *   A sorted array of tids and 0 if the root is a parent.

   */

  private function getParentTids($term) {

    $parent_tids = [];

    foreach ($term->get('parent') as $item) {

      $parent_tids[] = (int) $item->target_id;

    }

    sort($parent_tids);

    return $parent_tids;

  }

  /**