Drupal 8: Custom path aliases exported to code.

mymodule.install

<?php

/**
 * @file
 * Install, update and uninstall functions for my module.
 */
use Drupal\Core\Database\Database;
use Drupal\Core\Language\LanguageInterface;

/**
 * Implements hook_install().
 */
function mymodule_install() {
  mymodule_import_aliases();
}

/**
 * Add custom path aliases.
 */
function mymodule_update_8001() {
  return mymodule_import_aliases();
}

/**
 * @return array[]
 *   An array of arrays with the following structure:
 *   @code
 *     array(
 *       array(
 *         'source' => '/calendar',  // source path
 *         'alias' => '/agenda',     // path alias
 *         'langcode' => 'nl',       // language code for the alias (optional)
 *       )
 *     )
 *   @endcode
 */
function mymodule_get_exported_aliases() {
  return [
    [
      'source' => '/calendar',
      'alias' => '/agenda',
      'langcode' => 'nl',
    ],
  ];
}

/**
 * Process the list of exported path aliases and save them to the database,
 */
function mymodule_import_aliases() {
  // Get a list of aliases to be imported.
  $aliases = mymodule_get_exported_aliases();

  // Save aliases.
  foreach ($aliases as $alias) {
    // Merge in defaults.
    $alias += [
      'langcode' => LanguageInterface::LANGCODE_NOT_SPECIFIED,
      'pid' => NULL,
    ];
    // Check if the path alias already exists.
    $query = 'SELECT COUNT(pid) FROM {url_alias} ua WHERE ua.alias = :alias AND ua.langcode = :langcode';
    $args = [':alias' => $alias['alias'], ':langcode' => $alias['langcode']];
    $count = Database::getConnection()->query($query, $args)->fetchField();
    // If the count is 0, we can proceed to insert the alias.
    if ((int)$count === 0) {
      mymodule_add_path_alias($alias['source'], $alias['alias'], $alias['langcode'], $alias['pid']);
    }
    // If the alias exists, remember the alias and its language.
    else {
      $existing[] = $alias['alias'] . ' (' . $alias['langcode'] . ')';
    }
  }

  // Return a message about existing aliases. This is not necessarily a problem,
  // so we do not throw an exception.
  if (!empty($existing)) {
    return t('Aliases %aliases were not imported because they already exist.', ['%aliases' => implode(', ', $existing)]);
  }
}

/**
 * Helper function to save a path alias to the database. Copied from
 * AliasStorage::save() and slightly modified.
 */
function mymodule_add_path_alias($source, $alias, $langcode, $pid) {
  if ($source[0] !== '/') {
    throw new \InvalidArgumentException(sprintf('Source path %s has to start with a slash.', $source));
  }

  if ($alias[0] !== '/') {
    throw new \InvalidArgumentException(sprintf('Alias path %s has to start with a slash.', $alias));
  }

  $fields = array(
    'source' => $source,
    'alias' => $alias,
    'langcode' => $langcode,
  );

  // Insert or update the alias.
  if (empty($pid)) {
    $try_again = FALSE;
    try {
      $query = Database::getConnection()->insert('url_alias')
        ->fields($fields);
      $pid = $query->execute();
    }
    catch (\Exception $e) {
      // If there was an exception, try to create the table.
      if (!$try_again = $this->ensureTableExists()) {
        // If the exception happened for other reason than the missing table,
        // propagate the exception.
        throw $e;
      }
    }
    // Now that the table has been created, try again if necessary.
    if ($try_again) {
      $query = Database::getConnection()->insert('url_alias')
        ->fields($fields);
      $pid = $query->execute();
    }

    $fields['pid'] = $pid;
    $operation = 'insert';
  }
  else {
    // Fetch the current values so that an update hook can identify what
    // exactly changed.
    try {
      $original = Database::getConnection()->query('SELECT source, alias, langcode FROM {url_alias} WHERE pid = :pid', array(':pid' => $pid))
        ->fetchAssoc();
    }
    catch (\Exception $e) {
      $this->catchException($e);
      $original = FALSE;
    }
    $fields['pid'] = $pid;
    $query = Database::getConnection()->update('url_alias')
      ->fields($fields)
      ->condition('pid', $pid);
    $pid = $query->execute();
    $fields['original'] = $original;
    $operation = 'update';
  }

  if ($pid) {
    // Do not clear cache or invoke hooks unlike the code in AliasStorage::save().
    return $fields;
  }
  return FALSE;
}