public function add_relationship($instance, $property = 'fields') {
    $this->__belongs_to = array('property' => $property, 'instance' => $instance);

    // Add relationship to all individual fields as well.
    foreach ($this as $item) {
      if ($item->has_property($property)) {
        $item->add_relationship($instance, $property);
      }
    }
  }