code: protected function extractRootFields(array $fields)
      {
          $result = [];
  
          foreach ($fields as $field) {
              $result[] = current(explode('.', $field, 2));
          }
  
          if (in_array('*', $result, true)) {
              $result = [];
          }
  
          return array_unique($result);
      }
docs: Extracts the root field names from nested fields.
  Nested fields are separated with dots (.). e.g: "item.id"
  The previous example would extract "item".
  
  @param array $fields The fields requested for extraction
  @return array root fields extracted from the given nested fields
  @since 2.0.14