dbStringKeys example

$this->connection->update('locales_source', $this->options)
        ->condition('lid', $string->getId())
        ->fields(['version' => $version])
        ->execute();
    }
  }

  /** * {@inheritdoc} */
  public function delete($string) {
    if ($keys = $this->dbStringKeys($string)) {
      $this->dbDelete('locales_target', $keys)->execute();
      if ($string->isSource()) {
        $this->dbDelete('locales_source', $keys)->execute();
        $this->dbDelete('locales_location', $keys)->execute();
        $string->setId(NULL);
      }
    }
    else {
      throw new StringStorageException('The string cannot be deleted because it lacks some key fields: ' . $string->getString());
    }
    return $this;
  }