docs: def add_sub_resource(self, relative_id, sub_resource):
          """Add sub resource"""
          existing_sub_resources = self.resources.get(sub_resource.RELATIVE_PATH_TEMPLATE, defaultdict(list))
          existing_sub_resources[relative_id].append(sub_resource)
          self.resources.update({sub_resource.RELATIVE_PATH_TEMPLATE: existing_sub_resources})